repeat exercise in heat and exertional alterations to ... · trotter, joshua adie, joshua...

REPEAT EXERCISE IN HEAT AND

EXERTIONAL ALTERATIONS TO

THERMOREGULATION (REHEAT)

Christopher Allan John Anderson Bachelor of Clinical Exercise Physiology

Submitted in fulfilment of the requirements for the degree of

Master of Philosophy

School of Exercise and Nutrition Science

Faculty of Health

Queensland University of Technology

2019

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) i

Keywords

Exercise, exercise physiology, temperature, thermoregulation, thermophysiology,

radiation, conduction, convection, evaporation, army, heat, skin temperature, core

temperature, heat stress, heat strain, heat illness, work-rest.

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) ii

Abstract

Background: When working in extreme heat, the Australian Army refers to Work

Tables that assume body core temperatures will on average peak at 38.5 °C at the end

of work, and that subsequent work bouts will have equal heat gain. The current

Continuous Work Table restricts personnel to two consecutive work and recovery

cycles in each 24 hour period, but does not consider potential sex-based differences

in core temperature or if successive bouts will result in different amounts of heat

gained. We examined the sex-based differences in peak body core temperature

following four work and recovery cycles in the heat.

Methodology: Fourteen males (M: 31.8 ± 7.9 yr; 178.3 ± 5.0 cm; 74.9 ± 8.0 kg,

54.4 ± 8.7 ml ꞏ kg-1 ꞏ min-1) and thirteen females (F: 31.5 ± 7.7 yr; 165.0 ± 6.5 cm;

58.3 ± 5.9 kg, 50.5 ± 5.9 ml ꞏ kg-1 ꞏ min-1) performed four successive bouts of

treadmill walking in 32.5 °C Wet Bulb Globe Temperature (WBGT). The bouts

alternated between 35 minutes (~550 W) and 55 minutes (~450 W), each separated

by 30-min seated rest at 28 °C WBGT. Participants wore standardised military

clothing including body armour and a helmet (10 kg). Women were tested on days 5-

8 of their menstrual cycle. Peak heart rate (HR), rectal (Trec) and 4-site mean skin

temperature (Tskin) were compared across exercise periods. Statistical analyses were

conducted using repeated measures ANOVA.

Results: Peaks in Trec were significantly different between the exercise bouts (Ex1-

Ex4) (p < 0.001), but not between males and females (p = 0.163). Five males and

five females reached 38.5 °C by the conclusion of the fourth work bout. There was

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) iii

no difference in peak HR (p = 0.422) or peak Tskin (p = 0.336) between males or

females, nor between exercise bouts (HR: p = 0.194) (Tskin: p = 0.586).

Conclusions: Participants on average did not meet the assumed body core

temperature of 38.5 °C within four work and recovery cycles. These findings indicate

the current Continuous Work Table can apply to both females (within the early

phases of their menstrual cycle) and males across four work bouts.

Real World Implications: The outcomes of this study suggest that the current

guidelines are appropriate for both sexes, and may inform Australian Army decision

making regarding appropriate durations for up to four repeated bouts of work and

rest when working in extreme heat.

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) iv

Table of Contents

Keywords .................................................................................................................................. i

Abstract .................................................................................................................................... ii

Table of Contents .................................................................................................................... iv

List of Figures ......................................................................................................................... vi

List of Tables ......................................................................................................................... vii

Statement of Original Authorship ......................................................................................... viii

Acknowledgements ................................................................................................................. ix

Chapter 1: Introduction ............................................................................................ 1

Background ............................................................................................................................... 1

Purposes .................................................................................................................................... 3

Objectives and Null Hypotheses of Research ........................................................................... 3

Thesis Outline ........................................................................................................................... 3

Chapter 2: Literature Review ................................................................................... 5

Thermoregulation During Exercise ........................................................................................... 5

Physiological Responses to Exercise ...................................................................................... 11

Design and Use of Work-Rest Tables in Military and Occupational Activities ..................... 23

Phase-Based Thermoregulation Changes ................................................................................ 34

Summary ................................................................................................................................. 39

Research Questions and Implications ..................................................................................... 40

Chapter 3: Research Design .................................................................................... 41

Methodology and Research Design ........................................................................................ 41

Research Protocol ................................................................................................................... 42

Experimental Trial .................................................................................................................. 43

Data Collection ....................................................................................................................... 49

Analysis .................................................................................................................................. 52

Chapter 4: Results .................................................................................................... 54

Summary of Sessions .............................................................................................................. 54

Physiological Strain During Work .......................................................................................... 57

Physiological Strain During Rest ............................................................................................ 75

Chapter 5: Discussion .............................................................................................. 79

Work Tables & Repeat Exercise ............................................................................................. 80

Other Points of Interest ........................................................................................................... 89

Limitations and Generalisability ............................................................................................. 92

Future Research Directions ..................................................................................................... 95

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) v

Conclusions ............................................................................................................................. 95

References ................................................................................................................. 97

Appendices .............................................................................................................. 119

Appendix A Human Research Ethical Approval .................................................................. 119

Appendix B ESSA Pre-Exercise Screening Tool................................................................. 121

Appendix C Adverse Event Report ....................................................................................... 122

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) vi

List of Figures

Figure 1: Timeline of experimental trial session.................................................................. 48

Figure 2: Percentage of participants with core temperatures at or above threshold values ............................................................................................................. 57

Figure 3: Minimum and maximum Trec values for all participants in all work and rest periods .................................................................................................... 61

Figure 4: Minute-by-minute plotting of mean Trec temperatures of males and females across all exercise and rest bouts ..................................................... 62

Figure 5: Minimum and maximum heart rate values for all participants in all work and rest periods .............................................................................................. 64

Figure 6: Minute-by-minute plotting of mean heart rates of males and females across all exercise and rest bouts ................................................................... 65

Figure 7: Minimum and maximum Tskin values for all participants in all work and rest period ...................................................................................................... 67

Figure 8: Minute-by-minute plotting of mean Tskin temperatures of males and females across all exercise and rest bouts ..................................................... 68

Figure 9: Minimum and maximum PSI values for all participants in all work and rest periods .................................................................................................... 72

Figure 10: Metabolic heat production per kilogram body mass values for all participants in all work and rest periods ........................................................ 73

Figure 11: Minute-by-minute plotting of cumulative heat strain index of males and females across experimental trial .................................................................. 74

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) vii

List of Tables

Table 1: Participant biometrics ............................................................................................ 55

Table 2: Participant hydration measurements during trial ................................................... 55

Table 3: Baseline physiological and perceptual measurements ........................................... 56

Table 4: Physiological measurements between high responders and non during baseline .......................................................................................................... 58

Table 5: Trec measurements during work ............................................................................ 60

Table 6: Heart rate measurements during work ................................................................... 63

Table 7: Tskin measurements during work ............................................................................ 66

Table 8: Additional measurements made during work bouts ............................................... 71

Table 9: Trec measurements during rest .............................................................................. 75

Table 10: Heart rate measurements during rest.................................................................... 76

Table 11: Tskin measurements during rest .......................................................................... 76

Table 12: Additional measurements made during rest periods ............................................ 78

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) viii

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best

of my knowledge and belief, the thesis contains no material previously published or

written by another person except where due reference is made.

QUT Verified Signature Signature: _________________________

Date: ____20/12/2019 ___________

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) ix

Acknowledgements

I must acknowledge the contributions of a number of colleagues, friends, and family

who have all made their own mark on this research project, and to whom I am greatly

indebted.

First and foremost, I must thank my supervisors Dr. Andrew Hunt and Professor Ian

Stewart, both of whom have made this research project achievable and enjoyable

beyond my expectations.

I must acknowledge the contributions and assistance from the Australian Defence

Science and Technology group, and say thank you to Dr Denise Linnane and Dr Mark

Patterson for providing me a financial scholarship and means to enhance my

professional development as a researcher.

All of my love and most sincere thanks must go to my partner Tricia, for her unending

patience with me throughout the ups and downs of the last few years, and for her

tireless contributions to this research project. I say thank you to my mum and dad for

being a perpetual source of encouragement and enthusiasm and keeping me centred on

my goals. I must additionally say thank you to my mum and dad for allowing Tricia

and I to live with them the last several months as we work towards the next goal for

our life together.

Finally I would like to thank my colleagues in the office: Dylan Poulus, Michael

Trotter, Joshua Adie, Joshua Schnebele, Jessica Turner, Olumide Awelewa, Lewis

Fazackerley, James Clark and Karen Mackay. Without your continued encouragement

and advice, I firmly believe this project would look very different.

Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) 1

Chapter 1: Introduction

Background

Humans are capable of working in extreme climates due to their ability to adapt to changing

environments. To sustain homeostasis, a body core temperature between 36.5 °C and 37.5 °C

must be maintained. Body heat is constantly generated through metabolism and muscle

activity, even at rest. When exposed to extreme heat, the body must constantly dissipate heat

to the surrounding environment at a rate that equals the amount of heat gained (either from the

environment or by performing work, which increases metabolic heat)1,2 in order to avoid

hyperthermia and ensure thermal balance. When exposed to extreme cold, body heat must be

conserved and maintained to avoid hypothermia. This balance between internal heat generation

and external heat gain (in hot environments) or external heat loss (in cold environments) is

constantly in flux depending on many intrinsic (e.g. fitness, body morphology, sex) and

extrinsic (e.g. activity intensity, clothing, food and fluid intake) factors. Heat can be transferred

either to or from the surrounding environment to maintain this balance via heat transfer

pathways.

When thermal balance is disturbed such that heat gain exceeds heat loss, the excess heat will

be stored, and body core temperature will rise. If the amount of heat stored reaches certain

thresholds (such as 40.0 °C), a human will become hyperthermic and thermoregulation efforts

will fail to sufficiently cool the body down without external intervention3. Such interventions

can include ceasing work, seeking shade, rehydrating, cooling with fans, and applying wet

towels or cooling vests. Immediate hospitalisation and deep-body cooling techniques may be

required to avoid the most serious consequences of heat overexposure.


To enable work in extreme heat without the risk of heat overexposure, the Australian Army

refers to a set of Work Tables which limit the duration of work that staff can perform in

different climates. Work Tables are comprehensive lists of climates, work intensities, and

uniforms with corresponding guidance on work duration limits4. These tables are designed to

ensure work is stopped before there is a chance of heat-related injury. By using a Work Table,

a military commander can limit the risks of overexposing staff under their supervision to

hazardous environments, and therefore reduce incidence rates of heat injury in the field.

However, findings from a recent study have indicated that these limits may be too conservative,

and staff may be able to work for longer periods, or perform more work bouts without putting

themselves at further risk5. Additionally, the current Work Tables have not considered the

potential for differing physiological responses to heat between male and female staff6-11.

Evidence suggests that females can exhibit a shift in basal core temperature of up to +0.8 °C

when exercising during the luteal phase of their menstrual cycle, which may put female

personnel at risk when working in the heat in later phases of their menstrual cycle11-16.

However, research has found no difference in the absolute body core temperature responses of

women across the menstrual cycle compared to men when working in humid and warm

conditions, though this was conducted across single bouts of exercise at fixed intensities, not

repeated cycles of work and rest17-19. This study will investigate the effect of sex on core

temperature across multiple bouts of work and rest.

Current Work Tables are based on research that has predominantly been based on findings in

male participants, whereas military and occupational workplaces are steadily increasing their

recruitment of female personnel. This means current Work Tables are based upon literature

that has not examined thermoregulatory responses between sexes20-22.


Purposes

The primary aims of this thesis were to investigate the effects of repeated bouts of exercise in

the heat on body core temperature, and to explore any sex-based differences in these responses

between males and females.

Objectives and Null Hypotheses of Research

For the primary objectives of this interventional study, the primary objectives and null

hypotheses were as follows:

To evaluate whether current Australian Army work and recovery protocols for hot

environments can be increased to four consecutive work and recovery cycles without

resulting in excessive heat strain in healthy adults.

o H0: Peak core body temperatures will be the same across all four work bouts

To examine the sex-based differences in elevation and restoration of core temperature when

working in the heat with current work and recovery protocols.

o H0: Core body temperature responses will be the same between male and female

participants

Thesis Outline

This thesis is comprised of five chapters. Following the introduction to the general topic and

an outline of the primary aims and objectives of this research project (Chapter 1), a literature

review of the current body of research is presented (Chapter 2). Chapter 3 demonstrates the

design and methodology of the interventional study presented in manuscript format. Chapters

4 and 5 present the results, discuss the relevant findings from this research, provide

recommendations for future research, and provide research conclusions.


Chapter 2 introduces the current literature basis for this study pertaining to thermoregulation

during exercise, the design and use of Work Tables in military and occupational activities, and

the restoration of thermoregulation after exercise. These inform the background literature for

the first primary objective of this research project. This chapter also scopes the relevant

literature on sex-based differences in thermoregulation during and after exercise, the relevant

phase-based changes to thermoregulation, and the impact of the menstrual cycle on

thermoregulation during exercise. These findings inform the literature for the second of the

primary objectives for this research project. All these findings formed the basis for the

investigation undertaken as part of this thesis (Chapter 3).

Chapter 3 presents the design and methodology of the intervention used in this research project.

It describes the participant selection process, the research protocol, and the analysis and

calculations used for data interpretation. It outlines the investigation of core body temperature

changes across four repeat bouts of exercise in the heat (n = 29). The design of this intervention

is heavily informed by the findings of Chapter 2, and the results are presented in Chapter 4.

Chapter 4 summarises the results observed and the research completed throughout this thesis.

It details participant biometric values, baseline physiological and perceptual measurements,

and data recorded across the experimental trials. The statistically significant findings are

presented in several formats. The outcomes from this section inform the discussion in Chapter

5.

Chapter 5 provides an in-depth discussion of the findings from Chapter 4, particularly on how

the results of the investigation impact the primary research objectives. It also explores the

limitations and generalisability associated with the research outcomes and proposes a number

of areas that warrant further investigation. This section concludes the key research findings.


Chapter 2: Literature Review

Due to the extreme climates encountered at home and abroad, Australian Army personnel

spend a lot of time working in the heat. Overexposure to heat can have disastrous consequences

for humans, leading to significant impairment and even death. When working in extreme heat,

the Australian military have a number of safeguards in place to protect the health of their

personnel. This literature review introduces human body thermoregulation, current Australian

Army protocols, the physiology of male and female heat loss, and the key research questions

of this project.

Thermoregulation During Exercise

Whilst performing work (exercise), heat is generated in active muscles and exchanged with the

surrounding environment23. Heat must be dissipated to prevent elevations in core temperature

leading to dangerous levels of stored heat24,25. To maintain a safe core temperature in hot

environments, the generation and exchange of heat to the body (metabolic heat load) must be

balanced with heat outputs from the body to the environment26. Whilst working, the storage of

heat develops as the sum of heat lost and heat gained27. By factoring heat gained or lost through

heat transfer pathways and metabolic heat generation, an equation can be used to calculate the

rate of heat storage (Eq.1)1,27,28. For a stable core temperature, heat storage (S) must equal zero

(0).

𝐸𝑞. 1 𝑆 𝑀 𝑊 𝑅 𝐶 𝐾 𝐸

Where S = Rate of heat storage measured in Watts per square meter (W ꞏ m-2), M =

rate of metabolic heat production (W ꞏ m-2), Wex = external work performed on or by

the body (W ꞏ m-2), R = heat exchange through radiation (W ꞏ m-2), C = heat exchange


through convection (W ꞏ m-2), K = heat exchange through conduction (W ꞏ m-2), and E

= heat lost through evaporation (W ꞏ m-2).

The body transfers heat through four heat transfer pathways: evaporation, conduction,

radiation, and convection25,28,29. Evaporative cooling occurs because of the vaporisation of

water at the skin surface and the transfer of water vapour to the environment, determined by

air flow over the skin and absolute humidity. The vaporisation of 1g of water off the skin’s

surface releases 2426 J of latent heat energy, assuming 100% of the water is transferred to the

environment30,31. This vaporisation will only occur with a sufficient water pressure gradient to

enable diffusion of water from the skin to the ambient environment27. In hot dry conditions,

there is a high water vapour pressure gradient between the skin surface and the environment,

which provides a high potential for evaporative cooling. In contrast, in warm humid conditions,

the low water vapour pressure gradient between the skin surface and the environment reduces

the effectiveness of evaporative cooling due to the high saturation of water in the surrounding

air. Without a sufficient gradient to enable water diffusion, sweat will collect on the skin

surface, be wicked into clothing, or be dripped off and lose any cooling effect. Clothing also

plays a role by inhibiting effectiveness of evaporative cooling, as it reduces air flow over the

skin and insulates the body from convective heat exchange32. Evaporation is the most effective

method to counter increasing internal heat storage33-35, and, is estimated to transfer between

666 and 680 W per 1 L of sweat vaporised27,33, depending on humidity and wind speed.

Radiation transfers heat when either the skin or a nearby radiating body has a higher

temperature than the other, causing the cooler (with less heat energy) body to absorb heat34,36.

For example, a hot halogen bulb may have a greater temperature than skin, causing the body

to gain heat. When a body is exposed to sunlight, the radiant heat load upon that body arises

from direct, reflected, and reradiated heat such as from shiny or white surfaces37,38. Convection


transfers heat between a surface and a contacting mobile fluid or gas; in hot environments,

convection aids internal cooling by bringing heated blood close to the skin’s surface, allowing

thermal transfer21,22. In air temperatures below ~35.0 °C skin temperature is higher than air

temperature, so heat is lost by convection38. However, in air temperatures higher than ~35 °C

skin temperature is lower than air temperature, and convection adds to the heat load. Finally,

conduction transfers heat from one surface to an adjacent, cooler surface with which it is in

contact, and can be responsible for up to 70% of internal heat transfer from active muscles to

skin20,36.

Heat is held in the body as a function of its mass, the mean specific heat of the body tissues,

and its mean temperature38. The transfer of heat between the body and the environment occurs

by heat flow down gradients in ambient temperature and relative humidity through

independently acting physical processes38. As the transfer of heat is an exchange, the body can

dissipate heat to the environment in order to avoid overheating, but also can gain heat from the

environment (adding to the metabolic heat load). Environmental factors such as negligible air

movement, high humidity and/or ambient temperature may prevent adequate heat loss38. The

temperature gradient between the body and environment dictates which way heat will flow. If

the body is hotter than the environment, the body will cool until thermal balance is achieved;

however, if the environment is hotter than the body, the body will gain heat.

Physiological control centre and mechanisms to regulate body temperature

The human thermoregulatory system has been described as being more complicated than any

actual technical control system39. It contains multiple sensors (e.g. thermoreceptors in the skin),

multiple feedback loops (e.g. internal thermoreceptors feeding to the hypothalamus via afferent

neurons), and multiple outputs (heat transfer pathways stimulated by efferent neurons)40. The

heat transfer path from body core to body shell is influenced by internal (e.g. internal heat


generation by exercise) and external (e.g. originating from environmental heat or cold) thermal

disturbances41. External thermal disturbances are rapidly detected by thermoreceptors in the

skin, which convey the sense of increasing heat via afferent neurons to the brain. The

hypothalamus then sends impulses via efferent neurons from the central nervous system to

stimulate thermoregulatory pathways. This enables the thermoregulatory system to act before

thermal disturbances affect body core temperature. This “closed loop” of sensation and reaction

is controlled by the autonomic nervous system and does not require a high level of central

nervous system activation. It is important to note that the thermoreceptors in the skin respond

to an increase in temperature as well as the rate of change of temperature40.

Heat strain is the physiological changes that occur as the result of the presence of excessive

heat in the body. Excess heat stimulates activation of thermoregulatory mechanisms via the

autonomic nervous system, causing cutaneous vasodilation (dry heat exchange) and sweating

(wet heat exchange) to promote heat transfer35. This autonomic thermoregulation is controlled

by the hypothalamus, which controls different autonomic actions such as adjustment of40:

Heat production by shivering

Internal thermal resistance by vasomotion (control of skin blood flow)

External thermal resistance by control of respiratory dry heat loss

Water secretion and evaporation by sweating and respiratory evaporative heat loss

The body temperatures at which thermoregulatory mechanisms are stimulated are termed onset

thresholds, and the subsequent increase in activation of these mechanisms per unit (°C) increase

of body temperature is termed thermosensitivity. The autonomic responses may be triggered at

different thresholds of body/skin temperature, depending on internal and external variables.

For instance, increased respiration and skin blood flow may be stimulated at lower onset

thresholds during exercise in humid heat, whereas sweating and reduced cellular water reuptake


may be stimulated at lower onset thresholds during exercise in dry heat25. Onset thresholds can

also vary depending on sex, anthropometric measures, fitness, acclimatisation, and type of

work being done (load-bearing or non-load-bearing). The effectiveness of these mechanisms is

influenced by several intrinsic and extrinsic factors when undertaking work in extreme

environments, including hydration, acclimatization, sex, age, fitness, body fat, body size, diet,

previous heat illness, alcohol, and medications2,28,42,43.

Sweat glands are stimulated by sudomotor innervation via the sympathetic nervous system,

from hypothalamic input44. Sudomotor innervation activates muscarinic acetylcholine

receptors, which cause perspiration to occur45-48. To confirm the degree of hypothalamic

control in sudomotor activation and to identify onset threshold of sweating in mammals,

invasive studies have been conducted in Rhesus monkeys46. The researchers altered the

temperature of the hypothalamus and skin temperature of the monkeys to stimulate a sweat

response. They found that an increase in hypothalamic temperature increased sweat rate in a

positive and linear fashion. However, a lower skin temperature required a higher hypothalamic

temperature to stimulate a sweat response without affecting the slope of the relationship (cooler

skin requires a higher core body temperature to begin sweating). This indicates that the

generation of a sweat response is controlled by the integration of both skin and core body

temperatures. When the monkeys rested in 38 °C environments, their skin temperature was

~37.62 °C and sweat began at a hypothalamic temp of 38.46 °C. In cooler conditions (36 °C),

their skin temperature was ~36.82 °C and sweat began at a hypothalamic temperature of ~38.83

°C46.

The onset threshold of heat loss pathways in humans has been determined through non-invasive

methods, such as by monitoring core body temperature and identifying when sweat begins to

form whilst exercising at a fixed rate of metabolic heat production49-51. A study by Poirier et


al., (2016) monitored the whole-body skin temperature onset threshold and thermosensitivity

of sweat responses whilst cycling at 300 W and 400 W in 35 °C and ~20% relative humidity52.

They found that, when cycling at a fixed metabolic rate of 300 W ꞏ m-2, sweating began at an

onset threshold of 36.63 ± 0.18 °C, with a thermosensitivity of 512 ± 130 W ꞏ °C-1, whereas a

rate of 400 W ꞏ m-2 yielded an onset threshold of 37.02 ± 0.24 W ꞏ °C-1 and a thermosensitivity

of 520 ± 298 W ꞏ °C-1 52. This means that onset threshold and thermosensitivity of heat loss

pathways increase with intensity of exercise. These values significantly change following a 14-

day heat acclimation process, as outlined by Poirier et al. (2016).

Another study by Gagnon & Kenny (2011) investigated the onset thresholds for cutaneous

vasodilation and whole-body evaporative heat loss of men and women. They determined the

mean body temperature onset threshold for cutaneous vasodilation whilst cycling for 90

minutes at 50% VO2max, then later at a fixed rate of 500 W in 35 °C and ~12% relative

humidity53. In the 50% VO2max trial, they found no sex-based difference in the mean body

temperature onset threshold of cutaneous vasodilation (males: 36.70 ± 0.09 °C vs. females:

36.76 ± 0.09 °C) and whole-body evaporative heat loss (males: 36.67 ± 0.09 °C vs. females:

36.76 ± 0.09 °C)53. However, they did report greater thermosensitivity of whole-body

evaporative heat loss in men than women (males: 762 ± 56 W ꞏ °C-1 vs. females: 559 ± 75 W ꞏ

°C-1)20. In the 500 W trial, they found no difference in onset threshold of cutaneous vasodilation

(males: 36.65 ± 0.11 °C vs. females: 36.77 ± 0.08 °C) and whole-body evaporative heat loss

(males: 36.61 ± 0.11 °C vs. females: 36.77 ± 0.06 °C). Like the 50% VO2max trial, they reported

greater thermosensitivity of whole-body evaporative heat loss in men than women (males: 795

± 85 W ꞏ °C-1 vs. female: 553 ± 77 W ꞏ°C-1)20.


Physiological Responses to Exercise

In hot conditions, heat loss is initially facilitated by an increase in vasodilation, which increases

the flow of heated blood to the skin and raises the surface skin temperature. If this heat loss

pathway is insufficient to control core temperature, the body temperature will rise further and

sweating begins to enable heat loss by evaporation27,54.

Sweat release is facilitated through the body’s eccrine glands, which are distributed over most

of the body surface within the dermal layer. Numbers of eccrine glands vary from 1.6 to 4.0

million in an individual, and their number does not increase with age55. The greatest density of

thermoregulation-associated glands is on the palms of the hands and soles of the feet (~600

cm2), followed by the forehead, upper limbs, trunk, and lower limbs (~60 cm2)50,55-59. It has

been reported that women have a higher density and greater total number of eccrine sweat

glands than men60-62, though this has been disputed in other literature63. Eccrine sweat glands

are not affiliated with hair follicles, and their main purpose is to assist with thermoregulation54.

The sweat secreted from these glands is predominantly water, with traces of sodium chloride,

urea, lactic acid, and potassium chloride64.

Accurate measures of sweat production can be calculated through body mass changes

(factoring in food and fluid intake), ventilated capsules (electronic sensor within a controlled

capsule on the skin surface), technical absorbents (measuring change in mass of material), or

iodine-impregnated paper (marks left on paper from active sweat glands)59,65. A maximal sweat

rate can be estimated using equations and can range between 1.5 and 2.5 litres of fluid lost per

hour, theoretically providing between 1000 and 1700 W of heat loss27,54. Men typically sweat

more than women during moderate physical activity, with males sweating approximately 0.8

L ꞏ h-1 vs. approximately 0.45 L ꞏ h-1 for females55. Other studies have reported sweat rates of

> 3 L ꞏ h-1 66 in a highly trained male. Additional research has published an average maximum


sweat rate of ~1.4 L ꞏ h-1 49 amongst fit humans. However, these high sweat rates are

unsustainable for a period longer than 4 - 6 hours when working under heat stress, at which a

decline in the volume of sweat is observed44,49,67,68. The mechanism for the decline in sweat

rate during sustained heat stress is not clear, though it is theorised that prolonged skin wetness

can swell the tissue surrounding sweat ducts, occluding the ducts and slowing further sweat

secretion45.

Average sweat rate is reduced with age, with men and women over the age of 70 sweating less

than young (average age of 35) sex matched controls69. Additionally, age increases the onset

threshold for a whole-body sweat response by nearly 0.5 °C, with these effects being even more

pronounced in older women69.

The capacity for heat loss through sweat vaporisation far outweighs what can be realistically

achieved through other heat transfer pathways54. For acclimatised subjects, the whole-body

maximum sweat rate is approximately 20% greater than for non-acclimatised70,71. It has also

been demonstrated recently that the rates of whole body sweat production are not impacted by

relative intensity of exercise (percentage of VO2max), body surface area, body mass, or core

body temperature; instead, they are dependent on the evaporative heat balance requirement

(Ereq) necessitated from the environment expressed in watts72. This means assessments of

whole body sweat rate should be undertaken under a fixed Ereq and not any other fixed

measures72.

Interplay between environment, metabolic rate, and clothing

Compensated heat stress exists when the rate of heat loss to the environment at a higher level

of strain is in balance with heat production, so that the body maintains thermal balance27.

Uncompensated heat stress occurs when evaporative cooling requirements exceed the

environment’s evaporative cooling capacity (or clothing limits exposure of skin to the


environment), resulting in a progressive/continuous gain to body heat storage73,74. Excessive

heat strain (dangerously high core temperatures) can lead to dehydration, heat cramps, heat

syncope, heat exhaustion, heat stroke, and heat injury24,25,34,75,76.

Heat stress imparted on a body is a product of three factors: environmental conditions (wet-

bulb globe temperature; WBGT), work intensity, and clothing77. Performing work generates

substantial metabolic heat. As the work becomes more physically demanding, metabolic heat

production increases27,43. Further, wearing clothing or personal protective equipment (PPE)

whilst working also increases metabolic heat production and heart rate, and reduces the

effectiveness of heat transfer pathways by barring heat transfer and evaporation from the skin

surface28,42,78-81. Clothing increases the amount of mass on the body and restricts movement.

Clothing that adds mass can dramatically increase metabolic work rates82. In addition, treadmill

walking in bulky clothing can increase oxygen consumption by ~10% compared to the same

exercise in light clothing carrying the same weight80,83,84.

The heat strain imposed by clothing is a result of excess body heat storage which can be

attributed to the combined effect of three factors: clothing characteristics, environmental

conditions, and the level of physical activity. Two heat transfer parameters are impacted by the

presence of clothing: thermal resistance (resistance to dry heat transfer by conduction,

convection, and radiation, expressed as Rt in °C ∙ m2 / W) and evaporative resistance (resistance

to evaporative heat transfer from the body to the environment, expressed as Ret in kPa ∙ m2 /

W)42. Rt can also be expressed as clo, where 1 clo = 0.155 °C ∙ m2 / W 32. Clo is widely used to

represent the amount of thermal resistance clothing ensembles offer, and 1 clo equates to the

amount of insulation necessary for comfort whilst sitting in a room with air movement of 0.1

m ∙ s-1 at 21 °C and 50% relative humidity (or approximately one’s everyday clothing)85.

Thicker or bulkier protective garments such as coveralls have greater clo values, indicating a


greater level of thermal insulation to the wearer. A standard military uniform without body

armour would have a clo of 1.37 (Ret = 28.7 kPa ∙ m2 / W), and a torso and extremity ballistic

protection ensemble would have a clo of up to 1.63 (Ret = 43.7 kPa ∙ m2 / W)86.

Additionally, an increase in Rt and/or Ret through additional clothing reduces the rate of heat

loss (due to inhibition of heat transfer pathways), and therefore may increase the rate of heat

gain. However, when physical activity levels increase, greater mass on the body through

clothing may have a similar or greater heat strain effect than the clothing ensemble’s Rt/Ret42.

Therefore, reducing Rt, Ret and mass can significantly alleviate heat strain imposed by

clothing80.

Exertional heat illness

Exertional heat illness is a pressing concern in civilian and military activities. Exertional heat

illness presents in two forms: heat stroke and heat exhaustion. Heat stroke is a state of

thermoregulatory failure and must be treated as a medical emergency. It is the most severe

presentation of exertional heat illness and is associated with irreparable internal damage and a

high proportion of fatalities87-91. Its symptoms include hot and dry skin, rapidly rising core

temperature, collapse, loss of consciousness or neuropsychiatric impairment, and

convulsions92. It can present as a moderate to severe heat illness, and is broadly characterised

by organ and tissue injury associated with sustained high core body temperature (usually but

not always > 40.5 °C)75,92-94. Heat stroke requires immediate and appropriate medical attention,

including removal of the victim to a cool area and deliberate reduction of the rapidly increasing

body temperature38,95. Without these steps, heat stroke will be fatal. Additionally, persons with

a history of exertional heat illness (particularly heat stroke) are often at greater risk of recurrent

heat illness during strenuous physical activity due to the disturbances to thermoregulatory,

central nervous, cardiovascular, musculoskeletal, renal, and hepatic systems95-101. However,


long-term effects are reduced markedly if proper treatment is initiated within 10 minutes of

collapse102.

Heat stroke causes tissue and cell structure swelling and degeneration, and widespread

haemorrhages leading to organ congestion and failure91. Of all heat stroke cases in the U.S.

Army, 13% are associated with renal failure103; a case series of heat stroke deaths in the Israeli

army found renal failure in 100% of cases89, and kidneys have been found clotted with

macroscopic haemorrhage in 20% of heat stroke victims post-mortem91. Hepatic failure is also

observed in heat stroke, due to reduced liver blood flow associated with hyperthermia88,90,104;

66% of Israeli army heat stroke deaths had evidence of liver damage89. Additionally, the effects

of hyperthermia cause the central nervous system to dysfunction, resulting in respiratory

alkalosis (pH approaching 6.9 – 6.8; normal pH 7.35 – 7.4588,104), and reduced cardiac output

and falling central venous pressure (insufficient skin blood flow for thermoregulation, and

organ ischemia).

Heat exhaustion is a less severe heat injury than heat stroke, but it can be a preliminary step

before heat stroke. Heat exhaustion is characterised by clammy and moist skin, weakness or

extreme fatigue, nausea, headache, no excessive increase in body temperature, and low blood

pressure with a weak pulse. Without prompt treatment, collapse may occur. It is usually

manifested by an elevated core body temperature (typically < 40.5°C) and is often associated

with a high rate or volume of skin blood flow, heavy sweating, and dehydration76,105. Heat

exhaustion most often occurs in persons whose total blood volume has been reduced due to

dehydration but can also be associated with inadequate salt intake even when fluid intake is

adequate27. Lying down in a cool place and drinking cool (10-15 °C), slightly salted water

(0.1% NaCI) or an electrolyte supplement, will usually result in rapid recovery of the victim of


heat exhaustion. Salt-depletion heat exhaustion may require further medical treatment under

supervision.

Due to the severe nature of the consequences of heat stroke and exertional heat injury, extensive

research has examined their incidence rates in several populations. In South Australia, worker

deaths with heat as cause or a contributing factor were found to occur at a rate of 5.4% per

annum (3.6% male and 1.8% female) in 2009106. In other statistics, reported cases of heat illness

range from 612 cases (419 male and 193 female) per 1,000,000 (0.06%) Canadian workers

from 2004 – 2010107 to 38,392 cases (28,591 male and 9801 female) per 100,000 population

per year (2.5%) U.S. emergency department admissions to hospital from 2006 – 2010108. In

athletic populations, incidence rates of heat illness range from 0.00276% (0.00234 male and

0.0042 female) from 2005 – 2007109 to 2.21% (1.33% male and 0.88% female) per hundred

athlete exposures in 1978110.

Person-years account for the number of people in a study and the amount of time each person

spends in a study; 100 person-years contains the data of 100 people over 1 year. In the U.S

Army, rates of heat illness with medical intervention range from 0.21 males and 1.31 females

per 1000 person-years (1683 cases total) in 2014111 to 1.67 males and 2.63 females per 1000

person-years (2652 cases total) in 2011112. In the U.S. Army, there were a recorded 37 deaths

(0.3 per 1,000,000 soldier-years) due to heat illness between 1980 and 2002103. Stacey et al.,

(2016) identified 565 cases of exertional heat illness in the British Army from 2009 to 2013,

with an overall incidence rate of 13.4% per year or 38 hospitalisations per 100,000 personnel113.

Stacey et al., (2015) also reported on 389 heat illness cases in the British Army between 2007

and 2014 (4.4 per month)114. In the U.S Army, reported cases of heat stroke requiring medical

intervention increased from 311 cases (285 male and 26 female) per 1000 person-years in

2010115 to 417 cases (384 male and 33 female) per 1000 person-years in 2015116. In the French


Armed Forces, 182 personnel were hospitalised for exertional heat stroke between 2004 and

2006117.

The incidence rates of heat illness and heat stroke are significantly greater in armed forces than

in civilian and athletic populations111. There are many case studies and case series investigating

the probable aetiology of heat-related injuries in global military personnel. For further detail

on heat illness, heat stroke, and heat-related deaths, readers are encouraged to peruse Gifford

et al.’s (2019) systematic review and meta-analysis on these topics111.

Environmental factors

Several environmental factors contribute to the risk of heat illness. This can include the

environmental conditions (ambient temperature, relative humidity, air motion, and amount of

radiant heat from the sun76). In particular, environments with very high heat (e.g. > 28 °C

WBGT, > 80 °F118, > 30.1 °C119) tend to have higher heat-related mortality, but also typically

cool climates undergoing a heat wave when people are unaccustomed to high temperature

variability120-123. Hot and humid conditions most readily predispose people to exertional heat

illnesses, where the environmental temperature is higher than the body’s skin temperature and

therefore heat loss is entirely dependent on evaporation (which is then impeded by high relative

humidity, further increasing the risk of heat illness)124-126. It should be noted that the body of

literature on heat illness incidence and geographical location has been found to concentrate on

mid-latitude and high-income countries, and underrepresent the regions that are least able to

adapt to climate-based health risks and are most likely to experience extreme heatwaves (and

therefore have populations most at risk of death and illness from extreme heat)127.

The climate of the previous day and night (high WBGT indices on the previous day with sleep

in warm or non-air-conditioned locations is one of the best predictors of exertional heat illness

on subsequent days of exercise128). Other environmental factors can include barriers to


evaporative heat loss (uniforms that do not allow water vapour to pass from the skin to the

environment97,129-131), wearing a helmet, and excessive clothing or equipment (additional

clothing and layers may cause greater absorption of radiant heat and increase metabolic heat

generation through added weight76).

Inter-individual factors

There are several inter-individual factors that influence the risk of heat illness, including

overzealousness, poor physical conditioning, exercise intensity, increased body mass index,

and age. Overzealous athletes and military personnel are more prone to exertional heat illness

because they tend to override the normal behavioural adaptations to heat and ignore the early

warning signs of heat illness97,132. Untrained persons (those who are unfit, overweight, or

unacclimated) can rapidly reach a dangerous core temperature with less than 30 minutes of

high-intensity exercise (>1000 kcal ꞏ h-1)133,134. Particularly, Marine Corps recruits who had a

BMI of > 22 kg ꞏ m-2 and who ran 1.5 miles in over 12 minutes were found to have an eightfold

increased risk of exertional heat illness during basic training than those with a BMI less than

22 kg ꞏ m-2 and who could run 1.5 miles in less than 10 minutes134.

Another factor, exercise intensity, has the greatest influence on the rate of increase in core body

temperature133. High-intensity exercise results in a substantial amount of metabolic heat

production, which then produces a rapid rise in core body temperature135-137. As aerobic power

(VO2max) improves, the ability to withstand heat stress generally also improves89,97,138,139. It is

important to distinguish between exercise conducted at absolute rates of metabolic heat

production versus relative intensities of work. If two people with very different VO2max values

perform exercise at an identical rate of metabolic heat production (expressed in watts), their

core temperature responses will be very similar140. If these people performed exercise at a

relative intensity of effort (as a percentage of their VO2max values), their core temperature


responses will be significantly different, with the higher relative exercise intensity yielding a

greater core temperature response140.

People with an increased body mass index (and higher body fat percentage) are less efficient

at dissipating heat during exercise due to their smaller surface area to body mass ratio (and thus

decreased capacity to dissipate heat), and produce more metabolic heat due to their increased

body mass28,76. People with a body mass index > 27 kg ꞏ m-2 are more often affected by heat

exhaustion92. Similarly, people with greater muscle mass produce more metabolic heat and

have a lower surface area to mass ratio, contributing to a decreased ability to dissipate heat97,141.

Additionally, age can play a role in the development of heat illness. It is a consistently reported

finding that military personnel under the age of 21 (and with less than one-year total service

time) have the greatest incidences of heat injury142. Of all heat illness hospitalisations in the

U.S. Army from 1980 to 2002, 40% were less than 21 years of age, and 44% of all cases had 1

year or less of military service143. However, these incidence rates drop significantly as length

of service increases.

Intra-individual factors

There are a great many intra-individual factors which interrelate to increase the chances of

exertional heat illness in military populations144. These include heat acclimatization,

dehydration, recent illness, medications, sleep deprivation, and electrolyte imbalances. In the

first of these factors, heat acclimatization, repeated heat exposure over 10 to 14 days results in

physiological adaptations that enable the body to cope more effectively with thermal

stressors145-147. The adaptations include: increases in stroke volume and sweat rate, and

decreases in heart rate, core body temperature, skin temperature, and sweat salt losses75,133,148.

The rate of acclimatization is related to aerobic conditioning and fitness; in general, a better

conditioned athlete will acclimatize to the heat more quickly76.


Dehydration magnifies the core temperature responses to exercise in temperate and hot

environments, and this effect is observed with a fluid deficit of as little as 1-2% of body

weight149-153. A person is considered “at risk” of exertional heat illness at a urine specific

gravity of > 1.020 μG96. The magnitude of additional core temperature elevation ranges from

0.1 °C to 0.23 °C for every percentage of body weight lost151,154-156. During intense exercise in

the heat, sweat rates can be as high as 2 L ꞏ h-1; if the fluid is not replaced, large deficits will

result129. Water loss that is not sufficiently regained by a successive bout of work increases the

risk for exertional heat illness129,152,157,158. In hot climates, army personnel are typically

functioning at a state of moderate dehydration, unless prompted to drink104. Risk of exertional

heat illness and heat stroke rises substantially with inadequate nutrition and hydration. Of 182

French Armed Forces heat stroke hospitalisations, 15.3% of subjects reported that they were

dehydrated or fasted upon heat stroke onset117. Additionally, 6% of the cases reported alcohol

consumption the night before, potentially contributing to dehydration the day of the event117.

Electrolyte imbalances can present a component of risk for developing exertional heat

illness159. These commonly arise with the use of diuretics (e.g. medications for high blood

pressure, alcohol, caffeinated drinks), and can occur even in trained, acclimatized individuals

who engage in regular exercise and eat a normal diet160,161. Most sodium and chloride losses

occur through the urine, but people with high sweat rates (>2 L ꞏ h-1) and sodium concentrations

and those who are not heat acclimatized can lose significant amounts of sodium during physical

activity76.

Likewise, individuals who are currently or were recently ill are at increased risk for exertional

heat illness because of fever, dehydration, or medications97,129. Out of 179 heat casualties

during a 14-km run (over a 9 year period), 23% reported recent gastrointestinal or respiratory

illness162. Certain medications or drugs, particularly those with a dehydrating effect or those

that increase metabolic rate, also provide increased risk for exertional heat illness163-166.


Medications that have been suggested to have an adverse effect on thermoregulation include

stimulants, antihistamines, anticholinergics, and antispychotics166. Of 182 French Armed

Forces heat stroke hospitalisations, 6% reported that they had ear, nose, and throat diseases, or

viral gastroenteritis upon heat stroke onset117. Of 33 cases in the British Army which had

traditional risk factors for exertional heat illness, 18 subjects identified intercurrent illness prior

to their heat event114.

Lastly, sleep deprivation can be a common factor in heat-related illness, especially in military

contexts. In an Israeli Defence Forces case series on fatal exertional heat stroke, poor physical

fitness and sleep deprivation were found to be key components in 5 out of 6 deaths to heat

stroke89. Of 182 French Armed Forces heat stroke hospitalisations, 11.5% of subjects reported

that they were sleep deprived117. Out of 33 cases in the British Army which had traditional risk

factors for exertional heat illness, 11 subjects identified sleep deprivation prior to their heat

event114.

Heat illness in military contexts

It is accepted that the most likely contributing factors to the development of exertional heat

stroke are poor physical fitness and lack of heat acclimatization92. So how is it that military

personnel, who often are in good physical fitness and routinely have some degree of heat

acclimation, have higher incidence rates of exertional heat illness and heat stroke? Whilst the

U.S. military has reported a falling incidence of exertional heat illness, at the same time an

increase in occurrence of life-threatening exertional heat stroke has also been found103,142.

Most heat illness in military contexts happen during training, where local environments often

are not as severe as the climate in deployment locations. This is supported by several studies

indicating most military heat illness and heat stroke cases occur on local bases, and not overseas

in typically hotter environments. For example, Abriat et al., (2014) found 82% of exertional


heat stroke cases in the French Armed Forces occurred in metropolitan France117; similarly,

Stacey et al., (2015) found 65.4% of exertional heat illness cases in the British Army occurred

in the UK, and just under half of these cases occurred in winter114. In the U.S. Military, 70%

of all exertional heat illness cases between 1980 and 2002 occurred within the US103. In the

French Armed Forces, motivation was the primary intrinsic factor contributing to the onset of

exertional heat stroke in 182 cases117. Additionally, 15.4% of the cases had previously had an

episode of exertional heat stroke, meaning the victims had first-hand knowledge of the warning

signs and yet continued to exercise/work117. In the Israeli Defence Forces fatal heat stroke case

series, it was found that organisational factors largely contributed to heat stroke deaths. Six out

six deaths were tied to “physical effort unmated to physical fitness”, 5 out of 6 were tied to

training at the hottest hours of the day, and 4 out of 6 were tied to improper work/rest cycles89.

The events that led to the greatest occurrence of heat illness were scheduled training activities,

such as basic or assault training, followed by exercises and manoeuvres103. In the British Army,

62.8% of heat illness cases from 2009 to 2013 occurred during training, with only 10.6%

occurring during field exercises and 6.0% during operational deployments113.

The varied nature of military work makes predicting the length and intensity of exercise (and

thus the risk of heat illness) difficult. In civilian and athletic populations, heat illness typically

develops as a human overworked themselves during a standard workday or a competitive event.

As an illustration of the varied work environments when working in the military that have

resulted in fatal heat stroke, six cases are presented by the Israeli Defence Forces from between

1992 and 200289: a recently returned soldier recovering from a leg injury for 6 weeks underwent

a 5-km run with full combat gear, and lost consciousness and died of heat stroke; an elite soldier

navigated 40-km in 48 hours, slept only four hours, barely ate and drank minimal water, and

was found dead two hours after being directed to climb a cliff at noon; a soldier engaged in a

5-km run after 4-weeks rest and having had diarrhea two days prior, collapsed during the run


and died; an overweight (BMI = 30) newly recruited (two weeks prior) soldier completed a 5-

km night march with only four hours sleep, collapsed at the end of the march and died; two

special forces soldiers engaged in an intensive physical training manoeuvre after being several

weeks away from training, and after only four hours of sleep, both soldiers collapsed on the

second day at midday within 15 minutes of each other, and both died upon arrival at the nearest

hospital. Despite the fact that these were personnel who passed the fitness criteria to enter

military service, the varied nature of military work placed them at substantially greater risk of

developing heat stroke than in any other occupation.

There are a great many factors that expose military personnel to a greater risk of exertional heat

illness than other civilian and athletic populations when working in the heat. Accordingly,

substantial military research has been conducted into minimizing the risks to soldiers working

in the heat, and the outcomes have established global standards for preventing overexposure to

potentially hazardous environments.

Design and Use of Work-Rest Tables in Military and Occupational Activities

Quite often, military, athletic and occupational activities are undertaken in environments where

the heat is so severe that humans cannot tolerate exposure for extended periods167-170.

Additional task-specific requirements such as protective garments or intense physical activity

can further diminish a human’s capacity for work in hot environments. The risks of heat-related

illness are particularly high in military endeavours, where ground forces may be required to

undertake hard physical labour in bulky uniforms, with potentially degraded nutritional or

physical status, in extreme heat104. In combination with military personnel’s drive to

successfully complete the mission, successful heat stress management during military

operations is very difficult.


In conditions where military personnel will be exposed to uncompensable heat stress during

work, physiological adaptations to heat (such as from acclimation or aerobic fitness) can do

little to enhance performance171-173. Therefore, to ensure work gets completed, the options for

commanders are to reduce work rates (and work for longer), or to maintain the work intensity

and reduce work time (by incorporating rest breaks). The average metabolic rate of work is

closely linked to tolerance of exercise when wearing protective clothing in high heat stress

environments174. Therefore, to reduce the metabolic rate of work and increase tolerance time

of exercise, the intensity of effort can be lowered, or rest periods can be introduced (creating

exercise/rest cycles). The exercise/rest cycles are most commonly used in military and

athletic/occupational settings to extend exercise tolerance time in high heat stress

conditions104,175,176. However, a one-to-one ratio for exercise-to-rest does not suit all

conditions, as the optimal ratio varies depending on metabolic work rate, climate (WBGT),

clothing and equipment worn, hydration status, and acclimation177,178. Mathematical modelling

can predict the most appropriate exercise/rest ratios for a given set of conditions, and these

models are represented in Work Tables.

Military Work Tables

Work Tables align with evidence-based assumptions of core body temperature elevations

associated with heat illness onset, to manage risk of overexposure to heat stress179,180. These

tables are usually conservative to provide a margin of safety (expecting < 5% heat casualties),

which is expected to account for variability in the core temperature responses of personnel104.

The design of Work Tables is such that they reduce risk of heat-related injuries from

overexposure to hazardous work environments. Their implementation in a military context

stems from the countless number of operations which have ended with one or more personnel

severely impaired after working in a high heat stress climate144,181.


Currently, the Australian Army Work Tables assume a body core temperature elevation of 1.5

°C at the conclusion of work, from 37.0 °C to 38.5 °C77. The Work Tables assume that

personnel will reach a body core temperature of 38.5 °C by the end of the initial work bout,

and that equal heat will be gained in successive bouts of work. This limit is set as biophysical

modelling indicates a substantially greater risk of heat injury when operating at body

temperatures over 38.5 °C179,180. At a core body temperature of 38.5 °C, 20% of working

personnel indicate volitional exhaustion, whereas such fatigue rarely occurs at temperatures

lower than 38.5 °C168,169,182. Although 38.5 °C is the assumed core temperature at cessation of

exercise, normal inter-individual heat gain variations of ± 0.2 – 0.4 °C can occur, meaning

personnel can finish work anywhere between 38.0 – 39.0 °C183-185. Some individuals may even

see core temperatures greater than 39.0 °C due to innate variability. Therefore, work limits

must be set to minimise risk of harm to all troops including individual variability.

For example, a Work Table might indicate that, for a soldier wearing standard body armour

and operating at a metabolic heat production of 300 W in 32 °C WBGT, work should be limited

to 20 minutes with a rest of 40 minutes afterwards, repeated for up to five hours total. The

Australian Army use two Work Tables – one designed to advise on work and rest cycles per

hour, for up to 5 hours (Work-Rest Table), and one designed for longer, continuous, bouts of

work (continuous Work Table), after which a Recovery Table is used to determine the required

rest period before a second bout of work can be conducted. Which table to use is at the

discretion of command personnel, depending on the task required. The Work Table enables

many repeat bouts of exercise at a shorter duration with a rest specified according to exercise

intensity. However, the continuous Work Table allows for only two repeat bouts of work, each

separated by a rest bout (the duration of which is controlled by WBGT), after which no more

work can be done in that day.


There is some evidence to suggest current Work Tables in the Australian Army are too

conservative, to the point where personnel are finishing work substantially below the expected

38.5 °C core temperature77. Additionally, there is evidence to show that aerobically trained

individuals are capable and comfortable working at core temperatures in excess of 38.5 °C186-

188. This implies ground forces can continue to work for greater lengths of time than are

currently recommended by Australian Army Work Tables. Although the Work Tables have

been in use for a number of years, recent research has indicated that a core body temperature

in excess of 38.5 °C may not impede performance and can be tolerated for extended periods

without causing physiological damage186-192. A study by Ely et al., (2009) found no differences

in running velocity in 17 competitive runners when running at a core body temperature > 40

°C than when running at < 40 °C186. Older research by Christenen & Ruhling (1980) found

core temperatures to fluctuate between 38.9 – 39.1 °C in a female distance runner completing

a marathon in cool air temperatures189, though this was with a sample of one woman. Similarly,

Maron et al., (1977) found core temperature to plateau between 38.9 – 41.9 °C in two male

distance runners completing a marathon in cool conditions190. A more recent study by

Veltmeijer et al., (2014) found that 15% of 227 recreational runners developed a core body

temperature ≥ 40 °C after 15-km running in 11 °C, with no difference in race times compared

to cooler runners191. Byrne et al., (2006) found that, out of 18 heat acclimatized male soldiers

completing a 21-km road race in 26.5° C WBGT, all runners completed with peak body

temperatures > 39 °C, with ten > 40 °C and two > 41 °C, and were asymptomatic of heat

illness192. In military research, a study by Nolte et al., (2011) had 18 males marching for 25-

km carrying 26kg in 28.8 °C WBGT; they found six individuals had body core temperatures

between 39.0 – 40.3 °C, with all individuals asymptomatic for heat illness and all 18 males

completing the march187. Lee et al., (2010) found that, out of 31 male soldiers completing a 21-

km road race in 26.4 °C dry bulb and 81% relative humidity, 24 runners achieved core


temperatures > 39.0 °C, with seventeen ≥ 39.5 °C, and ten ≥ 40.0 °C, with no significant

difference found in running speed between the hotter and cooler groups188. A recent paper by

Hunt et al., (2016) investigated heat strain in 37 male soldiers completing a 10-km march

carrying 41.8 ± 3.6 kg in 23.1 ± 1.8 °C WBGT; out of 28 non-heat-exhaustion-symptomatic

soldiers, five did not complete the march and had core temperatures of > 39.0 °C, though these

five reported similar heat-related symptoms to the remainder who completed the march77.

The results of these studies are in contradiction with the current Australian Army Work Table

limits, which would have expected participants to present with symptoms of heat injury at these

core body temperatures. This implies there is a potential for Australian Army personnel to

continue work beyond what is currently permitted. The current Continuous Work Tables

assume core temperature at the end of the initial bout of work to be approximately 38.5 °C, and

for an equal amount of heat to be gained in subsequent work bouts, leading to a progressively

higher core temperature. This cutoff is set as, when an average Australian Army soldier reached

a body core temperature of 38.5 °C, normal inter-individual variation of ± 0.2 – 0.4 °C would

allow for the majority of individuals to complete work in the 38.0 – 39.0 °C range, with

approximately 5% exceeding 39.0 °C77. Currently, it is anticipated that an equal amount of heat

will be gained in each subsequent bout. This means a core temperature gain of +1 °C in the

first bout is expected to be matched by a gain of +1 °C in subsequent bouts. Likewise,

Continuous Work Tables assume an equal amount of heat will be lost in subsequent rest

periods. They expect a core temperature reduction of –0.5 °C in the first rest to be matched by

a loss of –0.5 °C in subsequent bouts. However, evidence indicates both patterns may not hold

true. Recent research suggests that core temperature elevations are highest in the first work

bout and heat losses are lowest in the first rest, whereas elevations are lowest in the final bout

and losses are greatest in the final rest5,53,193,194.


These findings suggest that the previously assumed association between core body temperature

elevation and heat-related symptoms should be re-examined for the purposes of determining

the optimal exercise-to-rest ratio for work productivity and personnel safety. Accordingly, the

restrictive nature of the current Work Table of the Australian Army has been called into

question, with attention being directed to whether the total number of work bouts could be

increased without substantially increasing the risk of heat-related injury77. Additionally, the

research base of the current Work Tables does not make considerations for sex-based

differences in thermoregulation. This is particularly of relevance as military settings currently

do not consider sex as a potential factor in heat illness incidence rates during work in the heat,

despite evidence to the contrary53.

Restoration of Thermoregulation after Exercise

After completion of work in a heated environment, heat loss responses in the body are abruptly

ceased as the heat production stimulus is removed, and two main changes to the

thermoregulatory mechanism occur. Firstly, the body sees a period of hypotension195,196. This

hypotension stems from the rapidly reduced cardiac output without an equally rapid reduction

in peripheral resistance (as blood pressure is a factor of cardiac output and peripheral

resistance). Secondly, sweating is rapidly ceased197. This is thought to occur as an attempt to

avoid unnecessary water loss through sweating. These post-exercise changes limit the body’s

ability to lose heat stored from exercise195,198. This elevation of core body temperature above

resting levels can be sustained for up to 90 minutes after completion of exercise5,199,200. The

duration required to lose heat gained during exercise typically must be longer than the period

of exercise, as evidence shows only 30 – 50% of this heat is lost during a rest of the same

duration (1:1)5,199.


The Work Tables utilised in the military are constructed from evidence-based assumptions

regarding the risk of heat related illness corresponding to an elevation of core body

temperature75,77. Military work limits assume an elevation in body core temperature up to 38.5

°C is attained at the end of each work bout77. However, there exists a disparity between the

Continuous Work Table and current literature regarding sufficient periods of rest. The allocated

rest component of these tables may not be of sufficient duration to return to pre-work baseline

measures, potentially increasing the risk of heat-related injury with each subsequent bout5,25,199.

However, the total heat gained in subsequent exercise bouts may not be identical to initial work

bouts and this will be addressed later.

Several studies have investigated the prolonged elevation in core temperature after a single

bout of exercise in the heat. Gagnon (2012) conducted two studies: in the first, 16 males

completed 30 minutes of cycling at 70% of their VO2max in 42 °C and recovered for 60 minutes

in 30 °C, and their core body temperature measured at the end of the recovery was elevated by

an average of +0.6 °C201. In the second, the same 16 males completed 120 minutes of cycling

at a fixed intensity of 120 W in 42 °C and 20% relative humidity; they recovered for 90 minutes

in the same environment, and their end core body temperature was elevated by an average of

+1.2 °C201. Similarly, Kenny et al. (1997) completed two studies. In the first, five men

completed 18 minutes of treadmill running at 75% VO2max in 24 °C and 20% relative

humidity202. They recovered for 20 minutes in the same environment, and their end core body

temperature was elevated by an average of +0.7 °C202. They used the same group of males

treadmill running (at the same intensity) and recovering for the same periods of time in 29 °C

and 50% relative humidity, and found their end core temperature was +0.6 °C202. Another study

by Gagnon et al. (2011) assessed 10 males cycling at 130 W for 60 minutes in 35 °C and 20%

relative humidity and recovered for 60 minutes at 25 °C and 20% relative humidity53. The

participants finished rest with an average end core temperature of +0.4 °C. Kenny (2008)


completed another study in which 6 males and 2 females cycled for 60 minutes at 70 W in 30

°C and 30% relative humidity199. They recovered for the same duration in the same conditions,

and participants’ end core temperature was elevated by an average of +0.2 °C199. Finally,

Vargas et al. (2018) studied 6 males and 6 females cycling for 60 minutes at 66 W in 24 °C and

43% relative humidity; they recovered for the same duration in the same conditions, and their

end core temperature was elevated by +0.2 °C on average203.

On the whole, these studies link the cessation of enhanced heat transfer post-exercise to

significantly elevated core temperatures and post-exercise hypotension for an extended

duration after a relatively short bout of exercise5,196,197,199,201,204. This has led to the assumption

that the elevation in core temperature is consistent in successive exercise bouts, and combined

with the elevated core temperature during recovery, the core temperature at the end of the next

bout of work would be much higher. This assumption forms the rationale behind this research

project. This would lead to excessively high temperatures after only a few work-rest cycles.

Whilst these findings are significant, the studies they came from were interested in single bouts

of exercise, not repeat bouts.

Although the core temperature does not rapidly return to a baseline after a single work-rest

cycle, recent research has indicated that the body is better able to lose heat in subsequent

exercise bouts, and therefore core temperature does not reach progressively higher peaks.

Kaciuba-Uscilko et al. (1992) studied 10 males completing 4 cycles of 30 minutes work / 30

minutes rest whilst cycling at 50% VO2max in 22 °C and 60% relative humidity194. They

recovered in the same environment and noted core temperature elevations between +0.6 °C and

+0.7 °C above baseline at the end of each rest period, despite peak core temperatures being

elevated further with each successive bout194. Kenny et al. (2009) studied six males and four

females, who cycled at 500 W of metabolic heat production in 30 °C and 30% relative humidity,


completing three cycles of 30 minutes work / 15 minutes rest5. The participants recovered in

the same environment, and their core temperatures were still elevated by between +0.2 °C and

+0.5 °C at end of rest5. However, whilst their core temperatures reached greater peaks in each

bout, they lost more heat in each successive work bout. Gagnon et al. (2011) completed two

studies, in which 10 men cycled at 130 W in 35 °C and 20% relative humidity and recovered

in the same environment53. In the first study, the men completed three cycles of 20 minutes

work / 20 minutes rest, and core body temperatures at end of rest measured between +0.4 °C

and +0.5 °C; in the second study, the men completed six cycles of 10 minutes work / 10 minutes

rest, and their temperatures measured between +0.2 °C and +0.3 °C at end of rest53. Similarly,

although participants core temperatures reached greater peaks during exercise, each bout of

work saw an increase in the rate of heat loss. Lastly, Smith (2013) observed 10 men walking

at 47% VO2max on a treadmill in 21 °C and 58% relative humidity193. The participants

completed three work rest cycles: the first cycle was 20 minutes work / 10 minutes rest, and

the subsequent two cycles were 20 minutes work / 20 minutes rest in the same conditions. The

participants’ core body temperatures at end of rest varied between +0.7 °C and +0.9 °C, and

again, during work their core temperatures reached higher values, but their rates of heat loss

were increased successively193.

This published evidence demonstrates a reduction in the increase of core temperature in

successive exercise bouts. These results stemmed from a greater rate of increase in whole-body

heat loss for a similar rate of heat production in the subsequent exercise bouts, in addition to

increased thermoeffector responses (sweat rate, skin blood flow, heart rate). Other studies

regarding heat storage and loss during intermittent exercise have reported similar alterations to

thermoregulation whilst exercising and at rest, namely increases in the absolute rates of heat

loss in each successive bout of work and rest after the first41,135,167. Therefore, the assumption

of increased risk due to a progressive elevation in core temperature is inaccurate and


unnecessarily limiting, indicating that more than 2 successive bouts of work in the heat may be

possible without an excessive increase in core temperature.

Additionally, the current work-rest protocols do not consider the differing physiological

responses to heat between male and female staff. Current work-rest tables are built on research

that has predominantly been based on findings in male participants (as is evident in paragraphs

above), whereas military and occupational workplaces are steadily increasing their recruitment

of female personnel. This means current Work Tables are based upon literature that does not

reflect potentially different thermoregulatory responses between sexes20-22.

Sex-Based Differences in Thermoregulation During and After Exercise

Characteristics of the female body which may affect thermal stress-strain relationships include

hormone levels, anthropometric factors, and body composition. Hormonal variations

throughout the menstrual cycle may lead to increased core temperatures at rest and during

exercise. Anthropometric factors may make it harder for women to dissipate generated heat

during exercise. Body composition variations may cause women to metabolically generate

more heat whilst exercising at a similar intensity to men.

Physiology of the menstrual cycle

Menstruation is the cyclic, orderly sloughing of the uterine lining that occurs as a result of

hormonal interactions in females205. It is a complex, coordinated sequence of events involving

the hypothalamus, anterior pituitary gland, ovary, and endometrium206. It can easily be

perturbed by environmental factors such as stress, extreme exercise, eating disorders, and

obesity.

The physiology underlying this process begins in the hypothalamus. The hypothalamus

secretes gonadotropin-release hormone (GnRH), which stimulates the anterior pituitary gland


to secrete both follicle-stimulating hormone (FSH) and luteinizing hormone (LH)205,206. The

levels and timing of secretion of each gonadotropin is influenced by GnRH, feedback from sex

steroid hormones, and other autocrine and paracrine factors. These gonadotropins stimulate the

ovary to produce the steroid hormones estrogen or progesterone, as well as several key

autocrine, paracrine, and endocrine peptides. Although estrogen and progesterone have some

feedback at the level of the hypothalamus, the more dynamic feedback occurs at the level of

the anterior pituitary gland.

Normal ovulatory menstrual cycles occur at regular intervals (21 – 35 days between) and last

for < 7 days205,207,208. The ovarian cycle consists of the follicular phase, ovulation, and the luteal

phase, whereas the uterine cycle is divided into menstruation, the proliferative phase, and the

secretory phase. In ovarian cycling, the first day of the menstrual bleed is considered day 1206.

During the menstrual phase (typically days 1 – 7), the uterine endometrium undergoes changes

and is sloughed off because of low estrogen (E2) levels. This is a component of the ovarian

follicular phase (typically up to day 11), which is mainly controlled by the hormone estradiol.

Towards the end of the follicular phase, levels of LH and FSH from the anterior pituitary gland

rise, as do levels of E2 (which initiate the formation of a new layer of endometrium in the

uterus). As E2 levels peak, a surge in LH and FSH is noted (lasting for 24 – 36 hours), and

results in a release of an oocyte from the ovary. Upon ovulation, estrogen levels deplete.

After menstruation has completed and the follicular phase has ended, ovulation begins

(typically days 12 to 17). This is the most fertile phase, wherein the oocyte descends to the

uterus ready for fertilization. During this phase, E2 levels drop whilst progesterone (P4) levels

begin to rise, beginning the luteal phase.

The luteal phase represents the latter phase of the ovarian cycle (typically days 18 – 28) and is

mainly associated with P4 and core body temperature, as these measures are higher during this


phase than in other phases of the cycle. E2 also partially rises as an effect of cellular changes

within the ovaries. Approximately halfway through the luteal phase E2 and P4 levels will fall,

causing increased levels of FSH in preparation for the next cycle. Continued drops in E2 and

P4 trigger the end of the luteal phase, beginning menstruation and the beginning of the next

cycle.

Phase-Based Thermoregulation Changes

As a result of the menstrual cycle, basal core temperature has been found to shift upwards by

an average of 0.3 °C at rest during the luteal phase (compared to the midfollicular phase), and

this difference can increase up to 0.8 °C during exercise11-16. Elevated P4 levels and changes in

E2-to-P4 ratios within the luteal phase of the menstrual cycle are linked to this elevation in the

thermoregulatory set point209-211. This elevated core body temperature resets at the onset of

menstruation, remaining at this temperature throughout the follicular phase211. A short

temperature dip in the late-follicular phase (just prior to the luteal phase elevation) has been

reported, but was only observed in between 10% and 50% of menstrual cycles recorded10,212.

There are further measurable physiological differences in thermal responses of women over

the course of the menstrual cycle. Heart rate in the midluteal phase has been found to be on

average ~10 beats per minute greater than in the midfollicular phase11. Subjective responses

such as rating of perceived exertion are significantly greater when exercising in the midluteal

phase compared to exercise in the midfollicular phase11. Additionally, some authors have

reported that sweat onset is delayed during the postovulatory phase of the menstrual

cycle14,61,213, while others find no change15,62,214-216; still more report a greater sweat rate during

the mid-luteal phase7.

It has been reported that thermosensitivity for sweating and cutaneous vasodilation is increased

in the midluteal phase of the menstrual cycle during resting heat exposure, as well as during


exercise8,217. Evidence from Gagnon & Kenny (2011) indicates that the thermosensitivity of

whole-body evaporative heat loss response (whilst cycling at a fixed intensity of 500 W) is

significantly greater in men than women (795 ± 85 W ꞏ °C-1 vs. 553 ± 77 W ꞏ °C-1), but there

is no significant difference in onset thresholds for sudomotor stimulation (whole-body heat

loss) or cutaneous vasodilation20. Therefore, women demonstrate a lower whole-body

sudomotor activation at fixed rates of metabolic heat production (in non-load bearing exercise)

than men, due to the lower thermosensitivity of the response20. This agrees with previous

research indicating that females typically sweat less than males (30 – 40% less)47,218 during

weight-bearing exercise at a fixed external workload19,219-221 and at fixed percentages of

maximum oxygen consumption17,18,222-224.

When exercising in the heat, previous studies across the menstrual cycle have found no changes

in female cardiovascular responses (heart rate and/or VO2) at 20%225, 30%9, and 60%226

VO2max. The major limiting factor when working in the heat is expected to be the elevated core

temperature during the luteal phase. Only one previous study has examined the effect of

possible thermoregulatory changes over the menstrual cycle on exercise time to exhaustion in

the heat, where they found a longer time to exhaustion in the early-follicular phase during light

intensity intermittent exercise225. This study was undertaken in uncompensable heat, and the

authors reported the expected higher core temperate for the participants in the mid-luteal phase

of the menstrual cycle at the start of exercise. They also found that there was no difference over

the menstrual cycle for the rate of increase of the core temperature during exercise and at

exhaustion. Because the participants’ starting core temperature was lower in the mid-follicular

phase and increased at the same rate as in the mid-luteal phase, it took a longer time to reach a

critical core temperature at exhaustion. Therefore, it can be assumed that any exercise

undertaken in uncompensable heat conditions will be shorter in duration in the luteal phase

than the follicular, due to the heightened starting core temperature.


On the whole, research seems to agree that there is no change in thermosensitivity or overall

heat tolerance over the menstrual cycle22,211. However, the alteration to resting body

temperature during the luteal phase may result in increased thermoregulatory and

cardiovascular strain and a decrease in prolonged exercise performance211. It is important to

note that thermoregulatory differences between men and women arise largely from

morphological variations, and not specifically from physiological differences due to sex. Body

morphology can vary through body composition, body surface area, body mass, and the ratio

of surface area to mass (SA/m). The SA/m ratio in particular has been found to explain 10-48%

of individual thermoeffector variance between men and women exercising in compensable

conditions, with sex only accounting for 5% of this variation3,227. For example, when

performing non-weight bearing activities (such as cycling at an absolute exercise intensity) in

warm/humid and hot/dry conditions, a high mass and high surface area, resulting in a low SA/m

ratio, leads to lower core temperatures responses and lower heat strain228.

Body composition

Body composition describes the relative ratios of fat and fat-free mass making up a body. When

referring to the heat gain differences between lean and fat tissues, specific heat is used. Specific

heat describes the amount of heat required to raise the temperature of 1 gram of a substance by

1 °C. The specific heat of adipose tissues (fat) is 0.4 Kcal ꞏ g-1 ꞏ °C-1, whereas the specific heat

of lean tissues (muscle, bone, and water) is higher at 0.8 Kcal ꞏ g1 ꞏ °C-1 229. Therefore, as fat

has a higher capacity for heat than lean tissue, when two individuals with equal mass are

exposed to an equal amount of heat stress, the person with a greater amount of fat mass will

have a larger core temperature increase. However, these observations have only been made

between groups of very lean (< 11% body fat) and obese (> 30% body fat) people230.


Women typically have a higher percentage body fat than men (21.9 – 29.5% vs. 10.9 –

22.3%)231, which affects their responses to both cold and heat22. It is understood that a “healthy”

woman will have roughly 6 – 7% more body fat than a “healthy” male matched for other

anthropometric factors, though some female elite athletes can be as lean as their male

counterparts231. This inherent sex difference can be visualised as Behnke’s concept of minimal

weight, which approximates solely lean mass in men, but is larger in women due to “essential

fat” stored in mammary tissue and elsewhere232. Additionally, women tend to have greatest fat

stored over the leg (gluteal-femoral region), whereas men have fat stored over the posterolateral

trunk (visceral depot)69,233,234.

A higher fat content also indirectly contributes to increased heat generation when exercising in

the heat, as the weight of fat tissue contributes to the metabolic cost of lifting and moving the

body26,60. Therefore, as body fat percentage increases, heat stress upon the body becomes more

difficult to sufficiently manage (due to increased metabolic heat generation), and the internal

heat strain load will increase. As women may typically present with slightly more essential

body fat than men, it can be expected that women will see greater core temperature increases

when exercising in the heat than men.

Body surface area

The dimensions of the body surface have an important effect on whole-body heat exchange, as

the absolute rates of heat transfer through evaporation, convection and radiation will be greatest

in people with the greatest total body surface area. Since attainment of heat balance relies upon

an equalization of heat production with an equivalent heat loss, individuals with larger surface

area will be able to sustain a higher rate of heat production while still remaining in heat balance

due to their larger heat loss potential36. Smaller individuals with a smaller body surface area

will therefore have a relatively limited ability to dissipate heat, and an impaired ability to safely


regulate core temperature at high ambient temperatures or during exercise. Additionally, a

smaller body surface area will result in a reduced ability to dissipate heat via evaporation when

compared to a larger body surface area. The same rate of whole-body sweat production over a

smaller body surface area will result in higher local sweat rates, reducing sweating efficiency

and the total amount of heat transferred224.

A larger body size (measured in m2) results in a lower body surface area to mass ratio, as there

is a relatively smaller increase in body surface area for any given increase in body mass.

Therefore rates of heat exchange per unit of mass are higher in lighter individuals when

working in high ambient temperatures than for heavier people235. A person with a smaller

surface area to mass ratio will see a lower rate of heat storage for an equivalent metabolic heat

load and will result in a smaller core temperature change 36,236. As women are generally smaller

than men and have a 10 – 12% greater SA/m ratio, they will typically show a greater rate of

heat storage during an equal amount of metabolic heat generation22. This is due to the rapid

heat gain enabled by a high SA/m ratio when exercising in extreme heat, which is poorly

tolerated by small persons due to their low thermal mass237,238

Body mass

Body mass serves two roles in heat exchange: first, the mass is the body’s internal heat sink

and contributes to thermal inertia; second, the energy cost of weight-bearing exercise is greater

in people with larger body mass, increasing metabolic heat production239. In weight-bearing

activities (such as running at a fixed speed), a person with a higher mass will generate a greater

metabolic rate than someone with less mass at the same intensity of effort. This is especially

pronounced whilst running in humid/warm conditions where heat loss pathways are already

maximally activated236. The implication of this is that heavier runners need to exercise at lower

speeds to match the same metabolic work rate as lighter runners in humid conditions240.


On average, women have less body mass than men (avg. 21.5 kg ꞏ m-2 vs. 24.5 kg ꞏ m-2) due to

their smaller body sizes241. Additionally, research indicates that obese individuals tend to have

a lower density of sweat glands compared to leaner individuals, as the fixed number of sweat

glands are spread over a larger area with a larger body size58. Therefore, as women typically

have lower body mass and smaller body sizes than men, body size differences will result in

females generating less heat compared to men during exercise in the heat.

Given the significant number of intrinsic and extrinsic factors contributing to exertional heat

illness, risk mitigation techniques must be employed when working in extreme heat. The

Australian Army Work Tables provide durations of work and rest to commanders when

conducting exercises in the heat, but the appropriateness of these durations has not been

assessed between sexes, nor along a longer-duration work day.

Summary

Currently, the Australia Army Instruction dictates that no more than 2 consecutive bouts of

fixed duration and rest (set by Continuous Work Tables) are to be performed per 24-hour period

under specific environmental conditions. This limitation may hamper the effectiveness of

ground forces who are limited to a few hours of work per day in hot conditions. However, this

limitation is based on the assumption that sequential bouts of work will result in the same

amount of heat being gained in each bout. However, recent research has found that subsequent

bouts of exercise result in reduced amounts of heat gained in each repeat bout after the first.

Additionally, the tables have been developed using data specific to males, and the work and

rest durations have not been checked with females. This research project will assess the current

continuous Work Tables and investigate whether the current work limits may be increased

without increasing personal risk.


Research Questions and Implications

1. To evaluate whether current Australian Army work and recovery protocols for hot

environments can be increased to four consecutive work and recovery cycles without resulting

in excessive heat strain in healthy adults

2. To examine the sex-based differences in elevation and restoration of core temperature

when working in the heat with current work and recovery protocols.

This research project may inform the Australian Defence Forces and other occupational groups

who work in hot environments on strategies to manage work and recovery durations over the

work day. It will also assist in ensuring current work and rest durations are appropriate for

males and females when working in the heat.


Chapter 3: Research Design

Methodology and Research Design

Participants

After ethical approval from the Human Research Ethics Committee (Queensland University of

Technology) (Appendix A), a sample of 14 males and 15 females between 18 and 45 years of

age were recruited for this study. Prior to commencement of the study all participants

completed a health screening questionnaire (Appendix B) and provided signed informed

consent. Contraindications for participation included any cardiovascular disease or injury,

previous musculoskeletal injury that would impede exercise performance (as identified by a

health screening questionnaire), current medications that may impact their ability to

thermoregulate, current pregnancy, or a BMI below 18.5 kg ꞏ m-2 or above 32.9 kg ꞏ m-2

(following Australian Army entry criteria)1. Female participants were eligible if they were

using combined monophasic (not biphasic or triphasic) oral contraceptive methods, or could

self-report a regular menstrual cycle for at least the previous 6 months6. Participants were

screened with a treadmill VO2peak assessment to ensure they met the minimum aerobic capacity

criteria for entry into the Australian Army, which is a VO2max of >38.1 mL ꞏ kg-1 ꞏ min-1. Priority

selection was given to participants with a VO2peak above 45.0 mL ꞏ kg-1 ꞏ min-1 to more closely

reflect the fitness levels of Australian Army Infantry personnel.

Height was measured with a stadiometer to the nearest 5mm, with shoes removed and

participants instructed to stand fully upright with full inhalation. Nude weight was measured

1 https://www.defencejobs.gov.au/joining/can-i-join/health-and-fitness


to the nearest 50 grams using digital scales (Wedderburn BWB-600, Tanita Corporation,

Tokyo, Japan).

Research Protocol

Participants attended a total of three sessions: a screening and VO2peak assessment, an

experimental trial session, and a body composition session. The VO2peak session was conducted

a minimum of 24 hours before the experimental trial session, and the body composition was

conducted within one month after the experimental trial.

Screening and assessment of aerobic capacity

During the screening sessions, participants were screened for eligibility into the study by

completing a pre-exercise health screening tool (Appendix B). The participants’ height and

clothed weight were measured, and a chest strap heart rate monitor was attached (Polar Team2,

Polar Electro, Kempele, Finland). Aerobic fitness was measured using an incremental test to

exhaustion on a motorised treadmill (StairMaster ClubTrack 2100 Treadmill, Nautilus Health

& Fitness Group, Louisville, USA). The participants undertook a 5-minute treadmill warmup

at a self-selected intensity, and verbally confirmed they were ready to begin the VO2peak test.

Expired gas measures were obtained using a metabolic cart (Parvo Medics TrueOne 2400,

Parvo Medics, East Sandy, USA) connected to a two-way T-shape non-rebreathing valve

(Model 2700, Hans-Rudolph, Shawnee, USA) and oro-nasal mask (Model 7450 Silicone V2,

Hans-Rudolph, Shawnee, USA). Concurrent measurement of metabolic energy expenditure

utilised a 6 L fluted mixed box within the calorimeter. Concentrations of oxygen (O2) were

measured using a paramagnetic gas analyser (error of ± 0.1%), and carbon dioxide (CO2) were

measured using an infrared gas analyser (error of ± 0.1%) within the Parvo Medics system.

Prior to each session the gas analysers were calibrated using gas mixtures of 4% CO2 and 17%


O2, and the pneumotach was calibrated using a 3 L syringe (error of ± 3%). Metabolic energy

expenditure was calculated from minute averaged values for VO2 and Respiratory Exchange

Ratios (RER)242. Participants began the VO2peak test at 1% incline and a speed correlating to 6-

8/20 on a Borg Rating of Perceived Exertion (RPE) scale243. Each minute, the treadmill speed

was increased by 1 km ꞏ h-1 until participants indicated they were running at 16-18/20 RPE.

The incline was then increased by 1% per minute until participants reached their perceived

maximum effort and ceased exercising. Expired gas analysis yielded the participants’ highest

minute averaged VO2 during the maximal effort, which was recorded as their VO2peak. If their

measured VO2peak met the entry requirements to the Australian army (>38.1 mL ꞏ kg-1 ꞏ min-1),

they were deemed eligible to continue with the study. Participants who did not meet this

threshold were not able to continue with the study.

Body composition

During the body composition sessions, participants had their height and clothed weight

measured, and their body composition assessed using a fan beam Dual-Energy X-ray

Absorptiometry (DXA) (Lunar Prodigy, GE Healthcare Lunar, Madison, USA) and analysed

using dedicated software (encore, version 9, GE Healthcare Lunar, Madison, USA).

Participants attended the scanning session hydrated and rested (no exercise the afternoon before

and the morning of the scan). The scans were conducted by a qualified and accredited DXA

operator. All scans were conducted within one month of the experimental trial date. The DXA

scanner provided assessments of participants’ fat mass (error ± 0.3%) and fat-free mass (error

± 2.5%)244.

Experimental Trial

In preparation for the test, participants did not engage in exercise, ingest caffeine or alcohol 24

hours prior to testing, and were instructed to consume at least 40 mL ꞏ kg-1 body mass of water


over the 24 hours before the trial. They were asked to consume a light meal and 500 mL of

water 2 hours prior to arriving at the laboratory. Pre-trial hydration status was confirmed by

urine specific gravity (USG) (PAL 10s, ATAGO, Tokyo, Japan) of <1.020245. If participants

provided a sample >1.020 they were given an additional 500 mL of tap water, which was

consumed 30 minutes before commencing the trial. The average USG was 1.011 ± 0.005, and

only two participants required an additional 500 mL of water before beginning exercise.

At the beginning of the experimental trial, participants were reminded of the trial procedures

and verbal consent to continue was obtained. The trials were conducted between December and

March (Summer) to account for seasonal variations in core temperature and acclimatisation,

and all trials began between 7am and 9am to control for circadian core temperature variations.

Participants provided a urine sample for hydration assessment. Once adequate hydration was

determined, participants undertook a nude weight in a private lockable room and self-inserted

the rectal thermistor to a depth of 12 cm, as marked with tape on the probe. They then re-

clothed, and a chest strap heart rate monitor and four skin temperature sensor thermocrons were

attached.

Female participants undertook the trial between days 5 and 8 of their regular menstrual cycle,

with day 1 being the beginning of menses. This date range was chosen as days 5 to 8 is when

females have a hormonal status most similar to males, and enables analysis of core temperature

variations based on morphological differences between sexes. Female participants self-

reported a regular menstrual cycle and predicted the appropriate date range for attendance.

During work, participants wore the Multi-Cam Uniform of the Australian Army (total thermal

insulation IT: 1.36 clo), their own shoes, and an Australian Army helmet and vest (Body

Armour covering 3299.5 cm2) totalling 10kg of weight. The helmet and vest were removed

during rest periods (resulting in a clo of 1.36 during rest), and participants were instructed to


remain seated for the duration of the rest period. Please refer to Figure 1 for a timeline of the

experimental trial.

At the beginning of the trial, participants remained seated for a 20-minute baseline period246 in

a thermoneutral environment (24.0 ± 1.3 °C, 56 ± 9% relative humidity, <0.1 m ꞏ s-1 air speed)

whilst resting measures were taken. Thereafter participants entered an environmental chamber

to start the experimental trial. The work and rest periods, and environmental conditions, were

determined in consultation with army stakeholders, and in accordance with the Army Standing

Instruction Continuous Army Work Table, and corresponded to Heavy work (> 500W) for 35

minutes and Moderate work (350 - 500W) for 55 minutes. From this, the corresponding work-

rest environments were selected to simulate real world training.

The experimental trial was conducted in an environmental chamber (dimensions 4 x 3 x 2.5m;

length, width, height) alternating between 32.5 °C WBGT (Air temperature: 38.8 ± 0.2 °C;

Relative humidity: 55 ± 1%; Wind speed: 1.5 m ꞏ s-1) during work and 28 °C WBGT (Air

temperature: 32.0 ± 1.1 °C; Relative humidity: 60 ± 6%; Wind speed: 0.5 m ꞏ s-1) during rest.

Ambient air temperature, relative humidity and Wet-Bulb Globe Temperature (WBGT) were

measured with a digital weather station (3M QuestTEMP 36, 3M, St. Paul, USA) recording at

1-minute intervals. WBGT was calculated using equation 127. Wind speed was measured with

a digital anemometer (Kestrel Pocket Weather 4000, Nielsen-Kellerman, Pennsylvania, USA)

once during each work period.

𝐸𝑞. 1 𝑊𝐵𝐺𝑇 0.7𝑇 0.2𝑇 0.1𝑇

Four consecutive work bouts were conducted on a motorised treadmill, alternating between 5.9

km ꞏ h-1 and 1% incline, with 10kg load (567 ± 143 W) and 4.5 km ꞏ h-1 and 1% incline, with

10kg load (439 ± 109 W). The wattages calculated correspond to “hard” and “moderate” work


in the Army Standing Instruction as calculated by the Pandolf Load Carriage equation (Eq.2),

assuming an average Australian Army soldier (82 kg) and a terrain factor of 1.0247:

𝐸𝑞. 2 𝑀 1.5𝑊 2.0 𝑊 𝐿𝐿𝑊

𝑛 𝑊 𝐿 1.5𝑉 0.35𝑉𝐺

Where M = metabolic rate, watts; W = subject weight, kg; L = load carried, kg; V =

speed of walking, m ꞏ s-1; G = grade, %; n = terrain factor (n = 1.0 for treadmill).

Participants were able to eat and drink freely during the trial, and the researchers verbally

encouraged participants to remain hydrated by drinking water and an electrolyte drink regularly

throughout the trial. Nutritionally appropriate food was provided in each rest period to assist

with fluid retention and to prevent fatigue. The available foods were recommended by an

accredited sports dietician to replenish carbohydrates and ensure adequate energy throughout

the trials, and included vegemite sandwiches, muesli and fruit bars, healthy wraps, and fresh

fruit. Although these foods are not a replication of a standard Army diet, this was done to

control for food and fluid-based alterations to thermoregulation (such as progressive

dehydration) and premature fatigue due to glycogen depletion.

All fluids and foods provided to and consumed by participants were weighed on a digital scale

(Propert 5 kg Slimline Glass Digital Kitchen Scale, McGloins-Supertex, Sydney, Australia).

Participants’ clothed body mass was measured at the end of each exercise period throughout

the experimental trial session using a digital scale (Wedderburn BWB-600, Tanita Corporation,

Tokyo, Japan).

The work bout was ceased if participants’ heart rate exceeded 90% of the measured maximum

obtained during the VO2peak screening session, if their core body temperature exceeded 39.0 °C

as per ISO 9886:2004248 and ACSM92 policy, or if they showed signs of heat-related illness.


Participants were then allowed to cease the trial completely if they wished, or to continue with

the next work bout after the 30-minute rest period. If a participants’ body core temperature

exceeded 38.5 °C at any point in time, all their data (including biometric values and hydration

measurements) were stored separate to the participants who did not exceed 38.5 °C for later

comparison. As the Work Tables assume that, on average, a core temperature of 38.5 °C will

be reached at the conclusion of work, it is important to ascertain whether any physiological

variables will be strong predictors of exceeding this threshold.

At the end of the trial, participants provided a final nude weight and were permitted to leave.

The pre and post nude weights were combined with the monitored food and fluid intake to

estimate sweat losses throughout the trial.


Figure 1: Timeline of experimental trial session


Data Collection

Physiological measurements

Core body temperature was estimated from rectal temperature (Trec) using a thermistor (YSI

400, DeRoyal, Knox, USA) self-inserted 12cm beyond the anal sphincter (following American

Society for Testing Material Standard F2668-16249) and recorded using a data logger (Squirrel

2020 series, Grant Instruments, Cambridge, UK) which was programmed to log every five

seconds. Samples were averaged per minute and analysed at the end of work and rest bouts.

The change in core body temperature, from rest to peak, as well as peak to lowest during rest,

was calculated.

Mean skin temperature (Tskin) was estimated using from four sites (neck, right scapula, back of

left hand, front of right shin) wireless iButton thermocrons (DS1922L-F50 iButtons, Maxim

Integrated, San Jose, USA) attached to four sites using a single piece of adhesive tape (Premium

Sports Tape, AllCare, Kumeu, New Zealand). iButtons were programmed before data collection

to log every 6-seconds at a resolution of 0.0625 °C via USB to a computer (DS9490R USB Port

Adapter, DS1402D-DR8 Blue Dot Receptor, Maxim Integrated, USA). Tskin was calculated

according to ISO 9886:2004 (Eq.3)248:

𝐸𝑞. 3 𝑇 0.28𝑇 0.28𝑇 0.16𝑇 0.28𝑇

Cumulative heat strain index (CHSI) was measured from minute-averaged heart rate and core

temperature values. It provides a numerical value describing the total amount of cumulative

heat strain a body undergoes, by comparing physiological measurements during work to

baseline.

CHSI was calculated using the following (Eq.4)250:


𝐸𝑞. 4 𝐶𝐻𝑆𝐼 ℎ𝑏 𝑓 ∗ 𝑡 ∗ 10 ∗ 𝑇 ∗ 𝑑𝑡 𝑇 ∗ 𝑡

Where hb = heart beats, fc0 = initial lowest heart rate (bpm), Trec = rectal temperature,

Trec0 = baseline rectal temperature (ºC) and t = time (min) from the onset of

measurement.

Sweat loss was calculated from changes in body mass by adding the participants’ initial nude

mass and fluid and food intakes, and subtracting their outputs and final nude mass, all in

kilograms.

Body surface area (BSA) was calculated using the following (Eq.5)251:

𝐸𝑞. 5 𝐵𝑆𝐴 0.007184 ∗ 𝑊𝑒𝑖𝑔ℎ𝑡 . ∗ 𝐻𝑒𝑖𝑔ℎ𝑡 .

Metabolic work rate was measured once over three minutes approximately halfway through

each exercise and rest bouts, using a metabolic cart (Parvo Medics TrueOne 2400, Parvo

Medics, East Sandy, USA) connected to a two-way T-shape non-rebreathing valve (Model

2700, Hans-Rudolph, Shawnee, USA) and oro-nasal mask (Model 7450 Silicone V2, Hans-

Rudolph, Shawnee, USA).

Metabolic heat production was calculated using the following formula (Eq.6)242:

𝐸𝑞. 6 𝐻𝑒𝑎𝑡 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑖𝑜𝑛 𝑊 352 ∗ 0.23 ∗ 𝑅𝐸𝑅 0.77 ∗𝑉𝑂1.84

∗ 1.84

Where RER = respiratory exchange ratio, and VO2 is measured in L ꞏ min-1

Physiological strain index (PSI) was measured continuously as a factor of core temperature and

heart rate. It describes physiological response to exercise on a scale from 0 to 10. Minute-


averaged core temperature and heart rate values were utilised to calculate PSI using the

following (Eq.7)252:

𝐸𝑞. 7 𝑃𝑆𝐼 5 ∗ 𝑇 𝑇 ∗ 39.5 𝑇 5 ∗ 𝐻𝑅 𝐻𝑅 ∗ 180 𝐻𝑅

Where Trect = rectal temperature at specific time point, 𝑇 = rectal temperature at

baseline, 𝐻𝑅 = heart rate at specific time point, and 𝐻𝑅 = heart rate at baseline

Perceptual measurements

Perceived intensity of exercise was measured using Borg’s Rating of Perceived Exertion scale

where 6 represents “no effort” and 20 indicates “maximum effort”243. Thermal sensation was

measured using a modified scale where 1 represented “unbearably cold”, 7 “neutral”, and 13

“unbearably hot”253. Thermal comfort was similarly measured on a modified scale where 1

represented “comfortable” and 5 “extremely uncomfortable”253. Both thermal sensation and

comfort were measured at 20-minute intervals during moderate work bouts, and 15 minute

intervals during rest and hard work bouts.


Analysis

The data is presented as means ± standard deviation unless otherwise stated. Group biometric

differences (age, height, body mass, body mass index, surface area to body mass ratio, VO2peak,

body fat percentage, and body surface area) between males and females were assessed by

independent samples t-test. Participants who exceeded the 38.5°C core temperature threshold

had their data compared to that of the participants who did not exceed this threshold via

independent samples t-tests. The primary data collected for analyses were Trec, HR, and Tskin.

Two-way repeated-measures analysis of variance (ANOVA) was used to assess the difference

in Trec, HR, and Tskin between the sexes, and across the work and rest bouts.

Trec absolute peaks, troughs (minimums), end of bout maximums, gains and losses, rates of

change and time delay from end of bout to absolute maximums were compared using two-way

repeated measures ANOVA across all four exercise bouts (Ex1-Ex4) and between sexes. Trec

values were comparable between all four bouts as each bout was assumed to reach a Trec value

of 38.5 °C irrespective of duration and intensity, based on the Army Work Tables.

Tskin and HR absolute peaks, gains (difference between minimum values at rest and maximum

values during the subsequent work bout) and losses (difference between maximum values

during work and minimum values during the subsequent rest bout) were compared using two-

way repeated measures ANOVA between exercise bouts of the same intensity (Ex1/Ex3,

Ex2/Ex4) and between sexes. Tskin and HR values were only comparable between bouts of the

same intensity as the duration and intensity of work significantly affected these measurements.

The within-subjects levels were Ex1-Ex4, R1-4, with a between-subjects factor of Sex. All data

were analysed using SPSS (SPSS version 25.0, SPSS Inc., Chicago, USA). All variables were

tested for normality by Shapiro-Wilks tests, and where the assumption of sphericity was

violated, the Greenhouse-Geisser correction was applied to adjust degrees of freedom and


increase critical values of the F-ratio. Statistical significance of the two-way repeated-measures

ANOVA was set to P < 0.05.

A retrospective (post hoc) power calculation using G*Power 3.1.9.4 software was performed

in order to determine if the experiment was sufficiently powered. For sex-based differences in

core temperature, heart rate, and skin temperature, over the four exercise periods, the effect size

f was calculated using Cohen’s d to be 0.51 with α set at 0.05 and a total sample size of 27.

Within G*Power, an F test ANOVA: Repeated measures, within-between interaction was

performed, resulting in a Critical F of 2.717. This project was sufficiently powered with the

number of participants recruited.

For differences between bouts for core temperature, heart rate, and skin temperature, the effect

size f was calculated using Cohen’s d to be 0.852 with α set at 0.05. Within G*Power, an F Test

ANOVA: Repeated measures within factors was performed, resulting in a Critical F of 2.717.


Chapter 4: Results

Summary of Sessions

Fourteen men and fifteen women participated in the trial. One female was stopped at the end of

the second work bout due to technical issues with the environmental chamber and was unable

to re-attend later to complete the trial. Another female suffered an unexpected adverse event

where she briefly stumbled on the treadmill 21 minutes into the second work bout, and the trial

was ceased for her safety (Appendix C). It was later determined that this participant had an

underlying illness when she attended the experimental trial session. For core temperature

measurements, if a participant did not complete a work bout to its full duration, that

participant’s data was excluded from the repeated measures analysis of variance. Therefore n =

14 men and 13 women for Trec, HR and Tskin. One female participant was included with a

VO2peak value of 39.5 mL ꞏ kg-1 ꞏ min-1. This participant was included in the study as her VO2peak

assessment was likely not a true maximal effort, with a reported maximal RPE of 15/20

(“maximal” > 18/20) and an RER value of 1.03 (“maximal” > 1.10), and she likely has a greater

aerobic capacity than was observed in this assessment.

Statistically significant differences were found in the biometric and anthropometric values

between the men and women in this trial (Tables 1 and 2). As recorded from the aerobic fitness

and body composition sessions, the men and women differed in height (t = -5.940, p < 0.001),

body mass (t = -6.118, p < 0.001), BMI (t = -4.044, p < 0.001), body surface area to mass ratio

(t = -5.264, p < 0.001), body fat percentage (t = 2.798, p = 0.010), and BSA (t = -8.219, p <

0.001). The participants also differed in several variables during the experimental trial: pre-trial

nude weight (t = -7.762, p < 0.001), post-trial nude weight (t = -7.519, p = 0.000), fluid intake

(t = -2.382, p = 0.025), sweat rate ( t = -2.132, p = 0.043), and absolute sweat losses (t = -2.132,

p = 0.043).


Table 1: Participant biometrics Men (n = 14) Women (n = 13) p-Value

Age (Years) 31.8 ± 7.9 31.5 ± 7.7 0.935

Height (cm) 178.3 ± 5.0 165.0 ± 6.5 < 0.001

Body mass (kg) 74.93 ± 7.96 58.31 ± 5.91 < 0.001

BMI (kg ꞏ m-2) 24.1 ± 2.1 21.2 ± 1.6 < 0.001

SA/m (cm2 ꞏ kg-1) 256.6 ± 12.7 282.9 ± 13.1 < 0.001

VO2peak (mL ꞏ kg-1 ꞏ min-1) 54.39 ± 8.69 50.49 ± 5.86 0.186

Body fat (%) 17.65 ± 6.94 25.00 ± 6.65 0.010

BSA (m2) 1.9 ± 0.9 1.6 ± 0.1 < 0.001

BMI = body mass index, BSA = body surface area, SA/m = surface area to mass ratio

Table 2: Participant hydration measurements during trial Men Women p-Value

USG 1.012 ± 0.005 1.012 ± 0.005 0.917

Nude weight (pre) 76.91 ± 6.33 58.45 ± 6.00 < 0.001

Nude weight (post) 77.13 ± 6.68 58.41 ± 6.22 < 0.001

Fluid intake (kg) 3.88 ± 1.12 2.93 ± 0.93 0.025

Food intake (kg) 0.48 ± 0.30 0.51 ± 0.39 0.832

Outputs (kg) 0.51 ± 0.16 0.53 ± 0.40 0.927

Sweat rate (kg ꞏ hr-1) 0.79 ± 0.25 0.62 ± 0.12 0.043

Relative sweat loss (g ꞏ kg-1) 0.053 ± 0.017 0.054 ± 0.013 0.812

Absolute sweat loss (kg) 3.94 ± 1.25 3.12 ± 0.62 0.043

USG = urine specific gravity


The wattage generated by the participants in the “moderate” and “heavy” work bouts accurately

represented the intensity of work described in the Australian Army Work Tables. The

participants generated an average of 439 ± 109 W during the “moderate” work bouts (expected

range of 350 – 500 W, calculated by the Pandolf Load Carriage), and 567 ± 143 W during the

“hard” work bouts (expected range > 500 W). There were no significant differences in the

absolute (men: 136.62 ± 51.3 W vs. women: 112.81 ± 56.16 W, p = 0.631) or relative metabolic

work rates (men: 1.79 ± 0.70 W ꞏ kg-1 vs. women: 1.93 ± 0.87 W ꞏ kg-1, p = 0.270) of the men

and women during the baseline measurements, although there were differences in the additional

load carried as a percentage of body mass (t = 6.228, p < 0.001), and the percentage of body

surface area covered by body armour (t = 8.101, p < 0.001). There were no significant

differences in any other physiological or perceptual measurement during the baseline resting

period (Table 3).

Table 3: Baseline physiological and perceptual measurements

Men Women p-Value

Trec (°C) 37.01 ± 0.27 37.04 ± 0.25 0.816

Heart rate (bpm) 64.7 ± 9.3 60.5 ± 12.0 0.326

Tskin (°C) 33.7 ± 0.8 32.7 ± 0.5 0.683

PSI (0-10) 1.4 ± 0.9 0.9 ± 0.7 0.137

Thermal Comfort (1-5) 1 1 0.623

Thermal Sensation (6-13) 7 ± 1 7 ± 1 0.563

Rating of Perceived Exertion (6-20) 6 6 0.345

Load carried relative to mass (%) 14 ± 1 17 ± 2 < 0.001

BSA covered by body armour (%) 17 ± 1 20 ± 1 < 0.001

Menstrual cycle day - 7 ± 1 -

PSI = physiological strain index


Physiological Strain During Work

Core temperature thresholds

Over the whole trial, ten participants (five males and five females) experienced a Trec above

38.5 °C, 35% of 29 participants (Figure 2). Of those ten, two males experienced a Trec above

39.0 °C, 7% of 29 participants. Participants who were observed to be approaching the 39.0 °C

threshold were instructed to cease work when they reached 38.9 °C; they immediately began

the next rest period and all 39.0 °C core temperature peaks were reached whilst at rest.

Figure 2: Percentage of participants with core temperatures at or above threshold values


No significant differences were observed in any biometric, anthropometric, or physiological

variable between the participants that reached and exceeded a core temperature of 38.5 °C

and those who did not before the trial or at rest (Table 4).

Table 4: Physiological measurements between high responders and non during baseline

< 38.5 °C (n = 19) > 38.5 °C (n = 10) p-Value

Age (Years) 32.7 ± 7.2 29.2 ± 7.8 0.239

Height (cm) 171.0 ± 7.3 173.3 ± 11.2 0.566

Body mass (kg) 65.05 ± 11.11 68.00 ± 11.54 0.508

BMI (kg ꞏ m-2) 22.4 ± 3.0 22.8 ± 1.5 0.648

SA/m (cm2 ꞏ kg-1) 271.8 ± 21.0 266.6 ± 15.5 0.470

VO2peak (mL ꞏ kg-1 ꞏ min-1) 51.89 ± 6.61 55.16 ± 10.00 0.299

Body fat (%) 22.35 ± 7.75 19.49 ± 8.10 0.361

BSA (m2) 1.78 ± 0.18 1.71 ± 0.20 0.692

USG 1.012 ± 0.005 1.011 ± 0.005 0.516

Nude weight (pre) 66.16 ± 11.83 68.97 ± 11.13 0.541

Nude weight (post) 66.23 ± 12.19 69.07 ± 11.20 0.545

Fluid intake (kg) 3.15 ± 1.22 3.56 ± 1.15 0.390

Food intake (kg) 0.50 ± 0.37 0.45 ± 0.31 0.710

Outputs (kg) 0.43 ± 0.25 0.76 ± 0.42 0.081

Sweat rate (kg ꞏ hr-1) 0.66 ± 0.21 0.72 ± 0.27 0.525

Relative sweat loss (g ꞏ kg-1) 0.052 ± 0.017 0.052 ± 0.016 0.894

Absolute sweat loss (kg) 3.50 ± 0.91 3.61 ± 1.34 0.811

Load carried relative to mass (%) 15.8 ± 2.7 15.1 ± 2.5 0.489

BSA covered by body armour (%) 18.7 ± 1.9 18.4 ± 2.1 0.726

Trec (°C) 36.97 ± 0.24 37.10 ± 0.26 0.213

Heart rate (bpm) 62 ± 12 63 ± 9 0.914

Tskin (°C) 32.5 ± 0.5 33.0 ± 0.7 0.095

PSI (0-10) 1.0 ± 0.6 1.0 ± 0.8 0.435

Thermal Comfort (1-5) 1 1 0.974

Thermal Sensation (6-13) 7 ± 1 7 ± 1 0.082

Rating of Perceived Exertion (6-20) 6 + 1 6 + 0 0.478


Core temperature

Peaks in Trec were significantly different between the four work bouts (Greenhouse F = 12.284,

p < 0.001, effect size 0.898 (Table 5) (Figure 3-4). Pairwise comparisons of all bouts indicate

that Trec peaks in work bouts 1, 2 and 4 were significantly lower than work bout 3. Positive Trec

rates of change were significantly greater in work bout 1 than bout 3 (Greenhouse F = 14.662,

p = 0.001), but were not different between work bouts 2 and 4 (Greenhouse F = 0.045, p =

0.834) (Table 5). Positive Trec changes (absolute gains) were significantly different between all

four work bouts (Greenhouse F = 39.227, p < 0.001). Lengths of time from the end of work to

peak body core temperatures were not significantly different across all four work bouts (F =

1.042, p = 0.387) (Table 5).

Across all four work bouts, there were no significant differences in peak Trec values between

men and women (F = 2.071, p = 0.163, effect size 0.869), sex and positive changes in Trec (F =

0.677, p = 0.418), nor sex and lengths of time from the end of work to peak body core

temperatures (F = 0.752, p = 0.404). There was no interaction between sex and positive Trec

rates of changes between bouts 1 and 3 (F = 0.200, p = 0.658), nor between bouts 2 and 4 (F =

0.594, p = 0.448) (Table 5).


Table 5: Trec measurements during work

Ex1 Ex2 Ex3 Ex4

Absolute peak values (° C) Men 38.20 ± 0.32 38.36 ± 0.34 38.49 ± 0.43* 38.33 ± 0.42 Women 38.05 ± 0.40 38.09 ± 0.34 38.33 ± 0.38* 38.15 ± 0.35 Core temperature changes from previous bout (° C) Men 1.19 ± 0.23 0.85 ± 0.31 0.71 ± 0.25 0.48 ± 0.19 Women 1.01 ± 0.29 0.65 ± 0.27 0.76 ± 0.25 0.57 ± 0.16 Time from end of work bout to peak Trec (minutes) Men 7.11 ± 3.92 6.34 ± 4.21 8.00 ± 5.29 5.56 ± 2.92 Women 5.50 ± 1.73 4.75 ± 2.63 6.50 ± 1.29 4.75 ± 2.22 Rate of change (° C ꞏ min-1) Men 0.0332 ± 0.0090 0.0126 ± 0.0052 0.0216 ± 0.0103 0.0097 ± 0.0051 Women 0.0314 ± 0.0128 0.0069 ± 0.0053 0.0223 ± 0.0103 0.0117 ± 0.0054

* Significantly different to all other bouts (p < 0.001) Core temperature changes calculated from rest bout minimum to subsequent work maximum Rates of change calculated across 10-minute sample during minutes 15-25 in Ex1/Ex3, minutes 30-40 in Ex2/Ex4


Figure 3: Minimum and maximum Trec values for all participants in all work and rest periods


Figure 4: Minute-by-minute plotting of mean Trec temperatures of males and females across all exercise and rest bouts


Heart rate

A significant difference was observed in peak HR, with bout 3 being greater than bout 1

(“heavy” work) (Greenhouse F = 24.172, p < 0.001, effect size 0.616), but no difference was

observed between bouts 2 and 4 (“moderate” work) (Greenhouse F = 1.783, p = 0.194, effect

size 0.525) (Table 6) (Figure 5-6). Positive HR changes (absolute gains) were significantly

greater in bout 1 than bout 3 (Greenhouse F = 5.915, p = 0.023), and greater in bout 2 than bout

4 (Greenhouse F = 18.697, p < 0.001) (Table 6).

There were no significant differences in peak HR values between men and women in bouts 1

and 3 (“heavy” work) (F = 0.3, p = 0.588, effect size 0.558), nor between bouts 2 and 4

(“moderate” work) (F = 0.551, p = 0.465, effect size 0.579). There were no significant

interactions observed between bouts 1 and 3 (“heavy” work) and sex for positive changes in

HR (F = 0.009, p = 0.926), nor between bouts 2 and 4 (F = 0.123, p = 0.728) (Table 6).

Table 6: Heart rate measurements during work

Ex1 Ex2 Ex3 Ex4

Absolute peak values (BPM) Men 138.0 ± 19.6 134.1 ± 19.1 148.4 ± 22.2 134.6 ± 21.1 Women 135.5 ± 21.6 127.4 ± 19.3 142.3 ± 19.3 130.4 ± 18.1 Heart rate changes from previous bout (BPM) Men 73.3 ± 14.2 58.5 ± 14.4 68.0 ± 13.5 47.9 ± 10.4 Women 75.0 ± 17.8 57.3 ± 11.8 70.0 ± 9.3 52.1 ± 9.3

Heart rate changes calculated from rest bout minimum to subsequent work maximum


Figure 5: Minimum and maximum heart rate values for all participants in all work and rest periods


Figure 6: Minute-by-minute plotting of mean heart rates of males and females across all exercise and rest bouts


Skin temperature

A significant difference was observed in peak Tskin, with bout 1 being greater than bout 3

(“heavy” work) (Greenhouse F = 16.286, p < 0.001, effect size 0.657), but not between bouts

2 and 4 (“moderate” work) (Greenhouse F = 0.586, p = 0.451, effect size 0.556) (Table 7)

(Figure 7-8). Positive Tskin changes (absolute gains) were significantly greater in bout 1 than

bout 3, (Greenhouse F = 10.459, p = 0.003), but not different between bouts 2 and 4

(Greenhouse F = 4.217, p = 0.051).

There were no significant differences in peak Tskin values between men and women in bouts 1

and 3 (“heavy” work) (F = 0.314, p = 0.581, effect size 0.553), nor between bouts 2 and 4

(“moderate” work) (F = 1.671, p = 0.208, effect size 0.610). No significant interaction was

found between sex and positive changes in Tskin between bouts 1 and 3 (F = 0.931, p = 0.344),

nor between bouts 2 and 4 (F = 0.050, p = 0.825).

Table 7: Tskin measurements during work

Ex1 Ex2 Ex3 Ex4

Absolute peak values (° C) Men 36.8 ± 0.6 36.5 ± 0.5 36.4 ± 0.6 36.4 ± 0.6

Women 36.8 ± 0.4 36.7 ± 0.4 36.6 ± 0.5 36.6 ± 0.5

Skin temperature changes from previous bout (° C) Men 4.1 ± 0.7 3.0 ± 0.8 3.4 ± 0.3 3.5 ± 0.4 Women 4.1 ± 0.4 3.1 ± 0.6 3.7 ± 0.9 3.3 ± 0.8

Skin temperature changes calculated from rest bout minimum to subsequent work maximum


Figure 7: Minimum and maximum Tskin values for all participants in all work and rest period


Figure 8: Minute-by-minute plotting of mean Tskin temperatures of males and females across all exercise and rest bouts


Additional measurements

Bout 3 was significantly greater than bout 1 (“heavy” work) for maximum PSI values

(Greenhouse F = 32.569, p < 0.001) (Figure 9) and ratings of perceived exertion (Greenhouse

F = 4.698, p = 0.04) (Table 8). There was no significant difference between sexes for bouts 1

and 3 maximum PSI values (F = 0.839, p = 0.369), nor ratings of perceived exertion (F = 0.265,

p = 0.611) (Table 8).There were no significant differences between bouts 1 and 3 (“heavy”

work) for relative metabolic heat production (Greenhouse F = 0.025, p = 0.876) (Figure 10) or

absolute metabolic heat production (F = 0.051, p = 0.824). There was no significant interaction

between sexes for bouts 1 and 3 for relative metabolic heat production values (F = 0.274, p =

0.274), but men displayed significantly greater absolute metabolic heat production than women

in bouts 1 and 3 (F = 6.881, p = 0.015).

Bout 4 resulted in significantly greater ratings of perceived exertion than bout 2 (“moderate”

work) (Greenhouse F = 9.412, p = 0.05), but no difference was observed between sexes for

bouts 2 and 4 (F = 0.257, p = 0.616) (Table 8).There were no significant differences between

bouts 2 and 4 (“moderate” work) for maximum PSI values (F = 0.226, p = 0.638), relative

metabolic heat production (Greenhouse F = 0.458, p = 0.505), nor absolute metabolic heat

production (F = 1.045, p = 0.317) (Table 8). There was no significant interaction found between

sexes for bouts 2 and 4 maximum PSI values (F = 3.102, p = 0.090), nor relative metabolic

heat production (F = 0.614, p = 0.441), but men displayed significantly greater absolute

metabolic heat production than women in bouts 2 and 4 (F = 6.556, p = 0.017) (Table 8).

There were no significant differences across all four work bouts for thermal comfort (F = 0.563,

p = 0.641), nor thermal sensation (F = 1.498, p = 0.222, nor between sex and all four work

bouts for thermal comfort (F = 0.106, p = 0.748) or thermal sensation (F = 0.408, p = 0.529)


(Table 8).There was no significant difference between men and women and CHSI results (p =

0.673) across the experimental trial (Figure 11).


Table 8: Additional measurements made during work bouts Ex1 Ex2 Ex3 Ex4 Maximum Physiological Strain Index (PSI) (0 – 10) Men 5.1 ± 0.9 5.5 ± 1.1 6.2 ± 1.5 5.4 ± 1.3 Women 4.7 ± 1.6 4.4 ± 1.4 5.7 ± 1.4 4.7 ± 1.2 Relative Metabolic Heat Production (W ꞏ kg-1) Men 7.93 ± 1.39 5.95 ± 0.94 8.01 ± 2.55 6.33 ± 1.78 Women 8.75 ± 1.48 6.72 ± 0.71 8.57 ± 1.37 6.70 ± 1.32 Absolute Metabolic Heat Production (W) Men 603.17 ± 117.71 452.69 ± 78.33 652.44 ± 75.24 512.33 ± 73.03 Women 506.20 ± 76.04 390.62 ± 52.54 499.65 ± 96.38 392.44 ± 91.46 Thermal Comfort (1-5) Men 2.3 ± 1.0 2.2 ± 1.0 2.3 ± 0.9 2.4 ± 0.8 Women 2.6 ± 0.7 2.5 ± 0.7 2.4 ± 0.8 2.3 ± 0.7 Thermal Sensation (6-13) Men 9.4 ± 0.9 9.2 ± 0.9 9.4 ± 0.9 9.5 ± 0.9 Women 9.2 ± 0.9 9.1 ± 1.2 9.5 ± 1.0 9.2 ± 0.5 Rating of Perceived Exertion (6-20) Men 11.3 ± 2.6 10.5 ± 2.8 11.2 ± 3.0 11.6 ± 2.6 Women 11.7 ± 2.5 11.6 ± 2.6 12.2 ± 2.4 11.8 ± 2.3


Figure 9: Minimum and maximum PSI values for all participants in all work and rest periods


Figure 10: Metabolic heat production per kilogram body mass values for all participants in all work and rest periods


Figure 11: Minute-by-minute plotting of cumulative heat strain index of males and females across experimental trial


Physiological Strain During Rest

Core temperature

Across all rest periods (but not baseline), there were significant differences in minimum Trec

values (Greenhouse F = 9.120, p = 0.001) (Table 9). Pairwise comparisons of all rest bouts

indicate that Trec minimum was significantly lower in bout 1 than in bouts 2, 3 and 4 (p < 0.05),

but bouts 2 and 3 Trec minimums were not significantly different from bout 4 (p = 1.0) (Table

9).

Negative Trec changes (measured work peak to subsequent rest minimum) were significantly

different between all rest bouts (Greenhouse F = 5.179, p = 0.003). Negative Trec rates of change

were not significantly different across rest bouts (F = 2.005, p = 0.121) (Table 9).

There was a significant difference observed between sexes, with men having higher minimum

core temperature values than women during rest (F = 4.621, p = 0.041). No difference was

observed between sexes for Trec changes (F = 0.005, p = 0.947), nor between sexes for Trec rates

of change during rest (F = 0.350, p = 0.559) (Table 9).

Table 9: Trec measurements during rest

R1 R2 R3 R4

Absolute minimum values (° C) Men 37.51 ± 0.28* 37.78 ± 0.24 37.85 ± 0.33 37.76 ± 0.31 Women 37.44 ± 0.21* 37.56 ± 0.18 37.58 ± 0.25 37.57 ± 0.35 Core temperature changes (° C) Men -0.69 ± 0.18 -0.58 ± 0.19 -0.64 ± 0.22 -0.56 ± 0.21 Women -0.61 ± 0.25 -0.53 ± 0.24 -0.75 ± 0.22 -0.58 ± 0.22 Rate of change (° C ꞏ min-1) Men -0.0171

± 0.0083 -0.0220 ± 0.0129

-0.0207 ± 0.0141

-0.0234 ± 0.0184

Women -0.0185 ± 0.0115

-0.0254 ± 0.0168

-0.0274 ± 0.0490

-0.0238 ± 0.0181

* Significantly different from other bouts (P < 0.05) Core temperature changes calculated from work bout maximum to subsequent rest minimum Rate of change calculated across final 10 minutes during rest


Heart rate

Each rest period was significantly different in minimum HR values (Greenhouse F = 19.345, p

= 0.001) and negative HR deltas (absolute losses) to each other rest period (Greenhouse F =

10.376, p = 0.001) (Table 10). There were no significant differences observed between sexes

for HR troughs at rest (F = 3.264, p = 0.083), nor between sexes for negative HR deltas (F =

0.221, p = 0.642).

Table 10: Heart rate measurements during rest

R1 R2 R3 R4

Absolute minimum values (BPM) Men 75.6 ± 11.8 80.4 ± 12.6 86.7 ± 14.7 68.8 ± 10.5 Women 70.1 ± 12.4 72.2 ± 13.0 78.4 ± 14.9 62.1 ± 10.1 Heart rate changes from previous bout (BPM) Men -62.4 ± 13.5 -53.7 ± 10.5 -61.6 ± 10.5 -65.9 ± 24.3 Women -65.4 ± 12.1 -55.1 ± 9.6 -63.9 ± 8.6 -68.3 ± 20.2

Heart rate changes calculated from work bout maximum to subsequent rest minimum

Skin temperature

Each rest period was significantly different for minimum Tskin values (Greenhouse F = 24.584,

p < 0.001) and negative Tskin changes (absolute losses) to each other rest period (Greenhouse F

= 17.502, p < 0.001) (Table 11). There were no significant differences observed between sexes

for Tskin troughs at rest (F = 0.006, p = 0.938), nor between sexes for negative Tskin rates of

change (F = 1.129, p = 0.298).

Table 11: Tskin measurements during rest

R1 R2 R3 R4

Absolute minimum values (°C) Men 33.5 ± 0.7 33.0 ± 0.6 32.9 ± 0.6 32.4 ± 0.6

Women 33.6 ± 0.6 32.9 ± 0.9 33.4 ± 0.8 32.0 ± 0.9

Skin temperature changes from previous bout (°C) Men -3.4 ± 0.6 -3.4 ± 0.3 -3.4 ± 0.3 -4.0 ± 0.7 Women -3.2 ± 0.5 -3.8 ± 0.9 -3.2 ± 0.7 -4.6 ± 1.0

Skin temperature changes calculated from work bout maximum to subsequent rest minimum


Additional Measurements (Table 12)

There were no significant differences observed across all four rest bouts for minimum PSI

values (Greenhouse F = 7.533, p = 0.001), minimum relative heat production values

(Greenhouse F = 0.659, p = 0.528), minimum absolute heat production values (Greenhouse F

= 0.594, p = 0.576), thermal comfort responses (Greenhouse F = 0.304, p = 0.749), thermal

sensation responses (Greenhouse F = 0.517, p = 0.613), nor RPE (F = 0.172, p = 0.915).

There were no significant interactions observed between sex and rest relative metabolic heat

production values (F = 0.035, p = 0.853), thermal comfort responses (F = 0.1.715, p = 0.202),

thermal sensation responses (F = 1.056, p = 0.314), nor RPE values (F = 0.162, p = 0.69).

A significant interaction was observed between sexes for rest minimum PSI means (F = 10.062,

p = 0.004), with men displaying greater PSI minimums than women across all rest periods.

Similarly, men displayed greater absolute metabolic heat production than women across all rest

periods (F = 5.072, p = 0.033).


Table 12: Additional measurements made during rest periods

R1 R2 R3 R4

Physiological Strain Index (0-10) Men 2.0 ± 0.7 2.3 ± 0.8 2.8 ± 1.1 2.3 ± 0.8 Women 1.1 ± 0.7 1.1 ± 0.7 1.6 ± 0.9 1.7 ± 0.8 Relative Metabolic Heat Production (W/kg) Men 2.23 ± 0.59 2.04 ± 0.72 2.21 ± 0.85 1.93 ± 0.58 Women 2.05 ± 0.78 2.41 ± 0.95 1.97 ± 0.87 2.10 ± 0.82 Absolute Metabolic Heat Produced (W) Men 162.82 ± 44.67 160.84 ± 48.92 174.21 ± 63.25 148.42 ± 39.36

Women 103.54 ± 58.02 121.57 ± 69.49 97.13 ± 59.24 105.92 ± 63.21 Thermal Comfort (1-5)

Men 1.3 ± 0.5 1.2 ± 0.5 1.3 ± 0.6 1.3 ± 0.4 Women 1.2 ± 0.5 1.2 ± 0.5 1.1 ± 0.3 1.1 ± 0.2 Thermal Sensation (6-13) Men 7.5 ± 1.1 7.1 ± 1.2 7.3 ± 1.3 7.1 ± 1.0 Women 7.1 ± 1.1 7.1 ± 1.0 6.8 ± 1.0 7.0 ± 0.9 Rating of Perceived Exertion (6-20) Men 6.3 ± 0.6 6.2 ± 0.4 6.4 ± 0.7 6.4 ± 1.3 Women 6.6 ± 1.4 6.4 ± 0.6 6.4 ± 1.1 6.3 ± 0.6


Chapter 5: Discussion

The aim of this research project was to evaluate the current continuous work table protocols

as specified for the Australian Army, and to determine if the limitations set on repeated work

bouts may be increased without increasing the risk of excessive heat strain to personnel. It also

evaluated the sex-based differences when undertaking work according to the current work-

table protocols and determined if there were any variations between male and female

participants completing the increased total work.

It was hypothesised that both men and women would be able to complete an increased total

duration of work that is currently recommended by the continuous Work Table without an

increased risk of heat illness, and that no major physiological differences would be observed

between men and women throughout the trials.

The key findings of this study include: (1) the work duration limits can be substantially

increased without further increases in body core temperature response and risk of heat illness;

(2) women within days 5 – 8 of their menstrual cycle and men exhibit no sex-based differences

in core body temperature response to repeat bouts of exercise in the heat. These findings agree

with our hypothesis, namely that the number of work bouts following the Australian Army

Work Table may be increased and that these tables need not have specific divisions for males

vs. females.


Work Tables & Repeat Exercise

The current Australian Army Continuous Work Tables are designed with the expectation that

a majority of personnel operating as per recommended work-rest ratios will reach a body core

temperature of 38.5 °C by the conclusion of each work bout. This threshold accounts for

individual variation in core temperature responses between ± 0.2 – 0.4 °C, expecting

approximately 5% of personnel to potentially reach core temperatures up to 39.0 °C by

completion of work77. This expectation accounts for the understanding that 5% of personnel

will present with heat exhaustion symptoms at core temperatures > 38.5 °C and a gradually

increased risk of heat stroke when > 39.0 °C. The continuous Work Table allows for two

consecutive bouts of work with a specified rest period in between, per twenty-four hours.

Core temperature elevations & risk of heat stroke (Trec > 39.0 °C)

This study monitored core temperature elevations of 29 individuals with comparable fitness

and body composition status to Australian Army personnel operating in the heat254,255. These

individuals followed the current Australian Army continuous Work Table in 32.5 °C WBGT

conditions for four consecutive bouts of work, twice the current limitation. It was assumed

that most participants would meet a body core temperature of 38.5 °C at the end of each work

period; however, this was not seen in this study, as only 35% of 29 participants reached 38.5

°C at any time during the trials. Of the 35%, five were males and five were females, and all

these participants had exceeded 38.5 °C by conclusion of the third work bout. Only 7% of

participants (two males) reached and exceeded 39.0 °C at any point during the experimental

trial (Figure 2). This agrees with the expectation that approximately 5% of personnel will reach

up to 39.0 °C by completion of work following the current Work Tables77.

The first male exceeded 39.0 °C twice during his trial: five minutes into the third rest bout,

and 53 minutes into the fourth work bout. He reached a peak core temperature of 39.13 °C 15


minutes into the third rest bout, and a peak of 39.19 °C 11 minutes into the fourth rest bout.

This participant reported no symptoms of heat illness and was reporting thermal comfort and

thermal sensations within normal ranges in both instances. It is important to note that this

participant consumed the least amount of fluid of all men across the experimental trial (average

3.88 ± 1.12 kg). Across the four work bouts he lost a cumulative 3.75 kg of fluid to sweat

(totalling 5% of body weight) and replenished 2.21 kg of fluid (totalling 3% of body weight)

and was therefore likely partially dehydrated (2% difference in fluid out vs. fluid in). This

participant was a current serving member of the Australian Defence Force as a field medic,

and had previously served several years stationed in Darwin, Australia. It is likely that his low

fluid intake is a trained habit from his experience in the field. Additionally, this participant

achieved the highest VO2peak assessment at 72.3 mL ꞏ kg-1 ꞏ min-1, which theoretically should

have correlated to a reduced heat gain during the experimental trial, though the opposite was

observed. He had a body fat percentage of 13.5% and a BMI of 22.4. The exact reason for this

participant exceeding 39.0 °C despite being well trained and experienced with operating in the

height is not known, though it is likely that his limited fluid intake played a major component.

The second male also exceeded 39.0 °C twice during his trial: six minutes into the second rest

bout, and three minutes into the third rest bout. He reached a peak core temperature of 39.06

°C eight minutes into the second rest bout, and a peak of 39.19 °C five minutes into the third

rest bout. Similarly, nil heat illness symptoms were reported by this participant, and he always

expressed thermal comfort and thermal sensations within normal ranges. In an opposite

direction to the other male participant exceeding 39.0 °C, the second male consumed the

second highest volume of fluid (5.63 kg of fluid), though his VO2peak was more modest at 51.0

mL ꞏ kg-1 ꞏ min-1. He had a body fat percentage of 23.9% and a BMI of 26.2. It is unknown

why these two participants with dissimilar body morphology and fluid intake both exceeded a

core temperature of 39.0 °C, and this may warrant further research to protect the wide variety


of personnel in the Australian Army. This unknown factor or factors are also discussed

elsewhere in literature, wherein no significant differences are found in the individual

characteristics of participants with high core temperature responses and those with normal

responses77,256,257

Of the participants who completed the trial (n = 27), 66% did not exceed the assumed thermal

threshold of 38.5 °C at any point during the trial, even after performing double the current

recommended number of work bouts by the continuous Work Table. There were no statistical

differences in the participants who reached or exceeded 38.5 °C at any point in time and those

who did not (Table 4). It is worth noting that the participants recruited for this study

represented a highly aerobically trained sample of people, who likely have some degree of

heat acclimation from exercising during the recent Brisbane, QLD summer. Any heat

acclimation resulting from recent training in a hot environment would correspond with lower

core and skin temperatures and heart rates at rest and during exercise in the heat, increased

sweat rates, and reduced RPE and thermal sensation responses258. This implies that fit males

and females may be able to work for greater lengths of time than are currently permitted by

Australian Army Work Tables.

Of all participants that reached or exceeded a core body temperature of 38.5 °C at any point

during the experimental trial (n = 10), none reported any symptoms of heat illness. Of the

participants that reached and exceeded a core body temperature of 39.0 °C (n = 2), neither of

them reported any symptoms of heat illness.

Since the tables assume an average person reaches 38.5 °C at the conclusion of each work

bout, it was expected that 50% of our participants would reach that value. However, this was

not observed in our participants. This is likely because our VO2max criteria provided a sample

of relatively fit participants who are similar in aerobic fitness (measured by VO2max) to


Australian Army personnel. Research indicates that as aerobic fitness increases, tolerance to

heat stress improves similarly, and people with greater VO2max values show lower elevations

in body core temperature for a standard absolute work intensity89,97,138,139.

Additionally, participants on average did not exhibit equal amounts of heat gained in

subsequent bouts of exercise. In fact, participants exhibited gradual reductions in the total

amount of heat gained during exercise in each subsequent bout after the first. This agrees with

previous research reporting that the absolute heat gain in repeat bouts of exercise is greatest in

the first work bout and lowest in the final work bout5,53,193,194. From this literature, it is

speculated that the cumulative heat gain is reduced in successive work bouts due to “priming”

of the heat loss pathways in each bout after the first. With each consecutive bout of work, the

body is more readily able to dissipate generated metabolic heat, and short-term adaptations to

efficiency of work reduce the total amount of metabolic heat generated. This priming is

presented as a shorter duration for onset of skin vasodilation and sweat secretion in each

successive bout of work, enabling heat transfer from an earlier point of time in later work

bouts5.

The change in core body temperature between each exercise bout was significantly different,

with less heat being gained in each subsequent bout after the first, and the amount of heat lost

in each rest bout being significantly different after the first. This substantiates the claim of a

gradual reduction in the heat gained in subsequent exercise bouts and a fluctuation in the

amount of heat lost in subsequent rest periods5,53,193,194. Between this finding of reductions in

absolute heat gain, and the previous finding of reductions in relative heat gain, it can be

suggested that the current Australian Army Work Table may be able to be revised to enable

more bouts of work. These findings suggest that the number of work bouts within the

Australian Army Work Tables may be revised without substantially increasing the risk of heat-

related injury.


Cardiovascular strain and risk of heat exhaustion

In this study, peak heart rate measurements between exercise bouts one and three (“heavy”

work) were significantly different, with bout three resulting in a higher heart rate than bout

one (Table 6). However, this difference was not observed between exercise bouts two and four

(“moderate” work), with both bouts resulting in similar peak heart rates. It is speculated that

the greater heart rate peak for bout three was due to accumulation of heat from the two prior

bouts of exercise, whereas the heart rate peak in bout one was recorded after 20 minutes of

exercise in the heat subsequent to 30 minutes of seated rest.

It is hypothesized that the greater heart rate peaks observed in later work bouts result from the

cardiovascular strain imposed by an increased core temperature, combined with continued

exercise in the heat. This finding is similar to other heart rate patterns reported in literature,

where significantly greater peak heart rates are observed in successive bouts of work and can

be termed as cardiovascular “drift”193,194,259, though other studies have failed to report any

increase in peak heart rate over repeat bouts of work5. This “drift” presents with increased

cardiovascular strain over repeat bouts of work, as the heart must increase cardiac output to

combat rising core body temperatures (through increased skin blood flow and reduced

peripheral resistance), whilst still providing oxygenated blood to the working muscles193. A

greater heart rate in bout 3 was likely observed as a consequence of cardiovascular drift, as it

was another “hard” work bout (with substantial physiological demand) with an elevated core

body temperature at onset (another significant physiological demand).

This increased cardiovascular strain of working at a high intensity in the heat and the

thermoregulatory requirements of a substantially elevated core temperature can potentially

increase the risk of heat exhaustion over the course of several successive work bouts260. This

is pertinent as demands for blood flow between the active muscles and the skin can result in a


competition for available cardiac output, especially when combined with the effects of

dehydration and hyperthermia214. We observed participants in the third (“hard”) work bout

reaching greater core temperatures and heart rates than in any of the other bouts (despite fluid

replacement and heat dissipation during the rest intervals), likely as an effect of this

competition for cardiac output. This matches findings reported in literature wherein greater

peak heart rate and core temperature values are found in successive bouts of work,

theoretically arising from a “slow drift upwards” in physiological variables over repeated

bouts of work at the same relative intensity194. This drift upwards was also reflected in the PSI

values obtained during the third bout of work, which were greater than those reached in the

other bouts. Therefore, in military training, consideration should be given to the intensities of

successive work bouts relative to the work done immediately before to avoid excessive

increases in the risk of heat illness. Commanders devising whole-day repeated drills may

consider the ordering of work tasks to avoid excess accumulation of heat, such as alternating

heavy workloads with light workloads or rest.

Maximum changes in heart rate between exercise bouts of the same intensity (bout 1 to bout

3 / bout 2 to bout 4) were significantly different throughout the experimental trial. This is

likely to have occurred as participants’ heart rates did not return to a baseline when at rest, as

part of their bodies’ process to continue dissipating heat gained from the previous work bout

(in line with literature-based expectations193,194,259). Therefore, participants began each

subsequent bout at a higher heart rate than the previous. Although the peak HR values were

significantly higher in bout 3 than bout 1 (but not different between bouts 2 and 4), the absolute

changes in HR likely stemmed from the increase in heart rate at each onset of work not

matching the increase in peak HR during work. This implies that other variables that are

commonly found to impact HR relative and absolute values may have been present, such as

fatigue198,261, hydration status151,152,168, and time of day262. However, our participants were


instructed to attend well rested and without exercise in the 24 hours prior, were assessed for

adequate hydration through USG, and all began the trial between 7am and 9am. This suggests

that other uncontrolled variables may have affected the results, such as genetics and training

history263. These unknowns present a potential area for future investigation.

Skin temperature findings

Peaks in Tskin were significantly different between exercise bouts 1 and 3 (despite both bouts

being of the same intensity and in the same climate) but not between exercise bouts 2 and 4

(Table 7). This finding is interesting as the climate between all four exercise bouts was

identical, and the only varied factor was alternating intensities of work bouts. The significant

reduction in absolute skin temperature gain between exercise bouts 1 and 3 implies an initial

spike in average skin temperature at onset of exercise; however, after this primary stimulation,

greater skin temperature loss is likely observed due to the priming of heat loss pathways3. This

finding of lowered skin temperature in successive bouts of work in the same climate is also

reported in other literature, though the absolute peak values vary5,53. It is likely that we

reported greater absolute peak skin temperature values due to the microclimate created by the

uniform worn, encapsulating the thermocrons and dramatically increasing skin temperature

compared to other studies with skin that is open to air flow. The uniform worn covered a

substantial percentage of the available skin surface area to dissipate heat from, hampering

effectiveness of sweat evaporation (particularly as the microclimate within the uniform can

reach high levels of humidity, further reducing potential for sweat evaporation) and local

cooling through conduction, convection and radiation36,42,264,265. As the trial uniform also

comprised of an impermeable vest covering between 14 – 17% of body surface area, it is likely

that the high skin temperatures observed in our study arose from the thermoregulatory

restrictions of the clothing266. The values obtained in this study are appropriate for an

encapsulating ensemble with helmet and vest, as athletic garments can yield skin temperatures


between 33 °C267 and 36 °C268 during exercise (depending on the ambient environment) and

protective garments can result in mean skin measurements above 36 °C269 very rapidly during

exercise in the heat.

Sex-Based Differences

The experimental design appropriately simulated work according to the current Australian

Army continuous Work Table, with participants completing up to twice the current prescribed

work duration limits. As previously mentioned, characteristics which may affect the thermal

stress-strain relationships within the female body include hormone levels, anthropometric

factors, and body composition.

Anthropometry and body composition linked variations

It is understood that women will typically present with a greater percentage of body fat, smaller

body size, less body mass, and a smaller surface area to mass ratio than men6,15,212,216,218. These

anthropometric and morphological variations are known to cause women to generate more

heat and find it harder to dissipate stored heat whilst performing exercise than men performing

the same work270.

The women in our study had, on average, a significantly greater body fat percentage, greater

body surface area to mass ratio, smaller body size, smaller body mass, smaller body surface

area, and lower BMI than our male participants (Table 1). Additionally, the female participants

had lower sweat rates and absolute sweat losses, and the body armour and helmet worn

weighed a greater percentage of their body mass and covered a greater percentage of their

body surface area than the male participants (Table 2). As anthropometric characteristics have

been classified as key factors influencing performance during exercise in the heat, it is likely

that these variables will have impacted our primary physiological measurements271-273.


The body surface area to mass ratios of the participants in our study are equivalent to those

reported in similar literature (males: 256.6 ± 12.7 cm2 ꞏ kg-1, females: 282.9 ± 13.1 cm2 ꞏ kg-

1; literature reported males between 258.9 ± 6.5 cm2 ꞏ kg-1 and 272.0 ± 4.0 cm2 ꞏ kg-1, females

between 276.0 ± 19.0 cm2 ꞏ kg-1 and 290.4 ± 7.4 cm2 ꞏ kg-1)224,228,273,274. It is well documented

that a large body surface area, large mass and lower body surface area to mass ratio provides

an “advantage” in hot dry and humid warm climates, particularly when comparing groups

completing work at the same relative intensity275. As the males in our study had larger body

surface areas, larger mass and lower body surface area to mass ratios than the females, their

morphological advantages would theoretically enable a more rapid dissipation of stored body

heat (and therefore reduced core temperature peaks) during the experimental trial228. However,

this was not seen, as there were no differences in peak core temperatures throughout the work

bouts between the men and women. Though the core temperature of our male participants may

numerically appear greater than that of the female participants, it is likely that this stems from

the significantly greater absolute rates of metabolic heat production observed in men than

women across all four work bouts (20% greater in bouts 1 and 3, 19% greater in bouts 2 and

4; Table 8). Therefore, the visually lower core temperatures of the women in our study are

more likely due to the metabolic rate difference than the body surface area to mass ratio

difference. This assumption is substantiated in the literature, wherein core temperature

variations between women controlled for the menstrual cycle and men exercising in the heat

can be explained by metabolic rate differences, rather than anthropometric variation228,273.

Additionally, this greater metabolic heat generation by the male participants may contribute

to why we saw two males (but no females) exceed a core temperature of 39.0 °C.

Menstrual cycle linked variations

It is important to note that these results were taken from female participants who were

exercised only during days 5 – 8 of their menstrual cycles. All other days of the menstrual


cycle were excluded from this study. It is anticipated that the heart rate would likely be

increased later in the menstrual cycle, by about ~10 beats per minute in the midluteal phase11.

This increase in heart rate responses across all work and rest bouts could theoretically enable

greater heat loss through increased peripheral blood flow, but this would likely have minimal

functional impact9,211,225.

There is evidence indicating no differences in absolute body core temperature responses of

women across the menstrual cycle when working in humid heat (after acclimation) compared

to men17-19; however, this conflicts with evidence that basal core temperatures can shift up to

+0.8 °C when exercising during the luteal phase11-16. If the latter observations are correct, then

the same absolute increase in heat gain during exercise may put female personnel at risk later

in the menstrual cycle when their core temperature at onset is higher11-16. However, the

research supporting that healthy and active women are not disadvantaged when working in the

heat compared to men is substantial, and supports our recommendation that the current Work

Tables need not be divided based on sex16. It is important to note that this body of literature is

within single bout, steady state work and does not investigate repeated work and cumulative

heat strain at various times across the menstrual cycle. While no difference was observed

during this trial with women at days 5 – 8 of their menstrual cycles, future work into repeated

bouts at different stages of the menstrual cycle needs to be investigated.

Other Points of Interest

Metabolic heat production

It is important to note that the absolute amounts of metabolic heat generated by participants in

this trial was matched to expected metabolic heat generation for “moderate” (439 W) and

“heavy” (567 W) work following current Work Table guidelines. No differences were

observed in metabolic heat production across the pairs of exercise bouts of the same intensity


(Table 8). This is in line with literature-based expectations, as each work bout is designed to

result in a core body temperature of 38.5 °C77. Therefore, absolute intensities of effort as

measured by gas sampling should present similar findings across all exercise bouts.

Rate of change

Positive Trec rates of change were significantly different between exercise bouts 1 and 3, but

not between exercise bouts 2 and 4 (Table 5). This is likely to have been observed as the first

work bout initiated the heat loss pathways, which were then already active from onset of

subsequent work bouts. This is similar to responses reported elsewhere, where the reduction

in total change in body heat (less total heat gained) was due to a greater rate of increase in

whole-body heat loss for a similar rate of heat production in the later bouts relative to the

earlier bouts5,194. The greater rate of change in core temperature we observed in later bouts in

this study matches the findings in other investigations during intermittent exercise in the heat,

where the rate of heat loss increases with successive bouts of work276,277. A similar protocol

to this project found a significant reduction in the “thermal inertia” (response time for

thermoregulatory pathways to effectively manage growing heat storage) over repeat bouts of

work, specifically lowering response time from 12.3 ± 2.3 min in the first work bout, to 7.2 ±

1.6 min in the second, and 7.1 ± 1.6 min in the third work bouts as measured through whole-

body calorimetry5. It is reported that this likely arises from a more rapid onset of local

sweating278 and skin vasodilation279 in successive bouts of work, in addition to alterations to

other nonthermal skin blood flow and sweat response factors280.

Rectal temperature is typically observed to be slow to respond to changes in core temperature,

though not significantly different from other core temperature measurement methods

(gastrointestinal pill or oesophageal thermistor)281,282. The rates of change during work bouts

observed in this study were similar to those reported in other literature for submaximal to


intense exercise (between 0.001 ± 0.006 °C ꞏ min-1 and 0.033 ± 0.011 °C ꞏ min-1; expected

~0.018 °C ꞏ min-1)281,283. The rates of change during rest are also similar to those reported

elsewhere for core temperatures during recovery from exercise (between -0.017 ± 0.011 °C ꞏ

min-1 and -0.024 ± 0.020 °C ꞏ min-1; expected between -0.010 ± 0.003 °C ꞏ min-1 and -0.029 ±

0.028 °C ꞏ min-1)282,284. As the participants wore a standardised Australian Army uniform

including a long-sleeve shirt, trousers, and running shoes, it could be expected that heat loss

at rest would be attenuated. However, as the uniforms were mostly saturated with sweat after

the first work bout, the damp uniform enabled heat transfer to the environment via conduction

and evaporation within expected ranges.

Sweat loss

Total sweat losses were estimated by adding food and fluid inputs, then subtracting any outputs

and the difference in pre-trial and post-trial nude weight (Table 2). These values were then

divided across the five hours of the trial to provide sweat loss per hour values. We observed a

significant difference in the total volume of sweat secreted per hour between the men and

women in our trial, with male participants closely matching expectations and the female

participants sweating slightly more than expected (men: 0.79 ± 0.25 kg ꞏ hr-1; expected ~0.8

kg ꞏ hr-1, women: 0.59 ± 0.16 kg ꞏ hr-1; expected ~ 0.45 kg ꞏ hr-1) 19,55,219-221. It has been reported

that trained men and women sweat more than untrained men and women, and that trained men

tend to sweat more than trained women224. As our sample represented a highly aerobically fit

group of men and women (of appropriate fitness to match Australian Army Infantry

personnel), these values are likely to mimic real-world sweat losses in the field.

There was also a significant difference in the amount of fluid consumed by the men and women

in this trial (men: 3.88 ± 1.12 kg, women: 2.75 ± 1.02 kg). All participants replenished at least

80% of the fluids lost to sweat throughout work and rest bouts, and no participant became


dehydrated during each bout (>2% body weight loss). This volume of fluid replaced is feasible

for soldiers, as Australian Army personnel typically carry ~10 kg of fluid when in the field285.

Limitations and Generalisability

The females recruited for this study were limited to a specific set of reproductive and

contraceptive criteria and were exercised only in a short window (days 5 – 8) of their menstrual

cycle. The potential for variations in the observed core temperature results was not

investigated at later points in their cycles. The females for this study participated during the

limited menstrual cycle window to reduce the potential impact of menstrual cycle-related

effects on body core temperature; this window was chosen as the 5 – 8 day period is when

females have a hormonal status most similar to males, and this enabled the researchers to more

appropriately assess core temperature variations stemming from morphological differences.

Further investigation is required to understand the potential impacts of different phases of the

menstrual cycle on core temperature during intermittent exercise in the heat.

Unfortunately, the study was not able to utilize the most accurate version of the current

Australian Army personnel uniform, as participants were instructed to wear their own

comfortable running shoes and carried less load than is typically expected. A standard

Australian Army member is expected to wear standard issue combat boots (an additional 2

kg), in addition to an approximately 40 kg load comprised of ammunition and tools, food and

water supplies, clothing, and personal possessions. The participants in this study only carried

an additional 10 kg of weight within a ballistic vest and helmet. The combat boots were not

worn in this study due to the range of sizes required, the discomfort associated with new and

unworn leather shoes, and the participants’ comfort when wearing unfamiliar and potentially

uncomfortable walking boots for an extended period. Wearing combat boots increases

metabolic work rate due to the increased weight at the end of the limb, reducing mechanical


efficiency and therefore forcing the body to work harder286. The full combat load was not

simulated as participants were not expected to be experienced in carrying heavy loads, and

this lack of adaptation was feared to cause overexertion and affect key physiological

measurements. In an ideal scenario, a similar study would be performed on Australian Army

personnel with access to their own broken-in combat boots and with experience in carrying

the required load. However, it is important to note that internal absolute metabolic heat

production rates were appropriately matched in this study to relevant “moderate” and “heavy”

metabolic rates stipulated in the Work Tables. The “moderate” work category assumes a

metabolic heat generation of between 350 W and 500 W, and participants in this trial averaged

an absolute metabolic heat generation of 439 W during the “moderate” work bouts. The

“heavy” work category assumes a metabolic rate greater than 500 W, and participants in this

trial averaged an absolute metabolic heat generation of 567 W during the “heavy” work bouts.

Therefore, even though our participants did not complete the trial in the most correct military

uniform of the Australian Army regarding weight and boots, the metabolic heat production

was appropriate to apply our findings to the current Work Tables.

Furthermore, the activities undertaken in this study were at a fixed wattage whilst conducting

work on a motorised treadmill. The current Australian Army Work Tables provide

recommendations for a wide range of work activities and is not specific to walking alone.

Although this study was completed with walking only, army staff regularly engage in other

tasks limited by the Work Tables, such as trench digging, engagement manoeuvres, training

exercises, and so forth. All of these tasks elicit different metabolic heat generation, and

therefore the findings from this study may not accurately represent heat gain potential across

all such activities.

The foods and fluids selected for this trial were not in direct mimicry of standard fare for

Australian Army personnel in the field. This was done to control for any diet-based variations


in core temperature, such as from the thermic effect of high-protein foods287 or the increase in

heat gain from dehydration. It is likely that, in the field, Army personnel would have limited

access to nutritionally appropriate foods and fluids, and therefore the findings from this study

may not accurately represent the hydration and nutrition status during military activities.


Future Research Directions

There is a lack of high-quality research into core temperature variations over repeat bouts of

exercise in females across the whole menstrual cycle. Future studies would ideally not limit

female recruitment by reproductive and contraceptive criteria, and also would exercise the

women across all days of the menstrual cycle. Additionally, future studies would recruit

specifically military personnel who had access to their own broken-in combat boots and with

experience in carrying the required load to simulate real-world military situations.

Conclusions

The key objectives for this study were: 1) To evaluate whether current Australian Army work-

rest protocols for hot environments can be increased to 4 consecutive work-rest cycles without

resulting in excessive heat strain in healthy adults; and 2) To examine the sex-based

differences in elevation and restoration of core temperature when working in the heat with

current work-rest protocols. Our findings concluded that peak core temperatures were

significantly different across all four work bouts, though core temperatures on average did not

exceed 38.5 °C. We also concluded that there were no significant differences in core

temperature responses between the male and female participants in this trial. These results

suggest that the current continuous Work Table of the Australian Army are appropriate for

both sexes, and may inform Australian Army decision making regarding appropriate durations

for up to four repeated bouts of work and rest and working in extreme heat.

97

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Appendices

Appendix A Human Research Ethical Approval

Dear Dr Andrew Hunt and Mr Christopher Anderson Ethics Category: Human ‐ Committee UHREC Reference number: 1800000438 Dates of approval: 1/08/2018 to 1/08/2019 Project title: Repeat Exercise in Heat and Exertional Alterations to Thermoregulation (REHEAT) Thank you for submitting the above research project for ethics review. This project was considered at a recent meeting by the Queensland University of Technology (QUT) Human Research Ethics Committee (UHREC). I am pleased to advise you that the above research project meets the requirements of the National Statement on Ethical Conduct in Human Research (2007) and ethical approval for this research project has been granted. Please find attached the Research Governance Checklist. Please ensure you address any items you identify as relevant to your research project. Note: If additional sites are engaged prior to the commencement of, or during the research project, the Chief Investigator (CI) / Project Supervisor (PS) is required to notify UHREC. Notification of withdrawn sites should also be provided to the UHREC in a timely fashion. The approved documents include: ETH_REHEAT_HREA_Output‐Form_1‐08‐2018 ETH_REHEAT_HREA_Project‐Protocol_1‐08‐2018 ETH_REHEAT_Email‐Recruit_1‐08‐2018 ETH_REHEAT_Flyer‐Recruit_1‐08‐2018 ETH_REHEAT_Information‐Sheet_1‐08‐2018 ETH_REHEAT_Consent‐Form_1‐08‐2018 ETH_REHEAT_MRIWalletAlert_1‐08‐2018 ETH_REHEAT_Screening‐Tool_ESSA_1‐08‐2018 Approval of this project from UHREC is valid as per the dates above, subject to the following conditions being met: < The CI/PS will immediately report anything that might warrant review of ethical approval of the project. < The CI/PS will notify the UHREC of any event that requires a modification to the protocol or other project documents and submit any required amendments in accordance with the instructions provided by the UHREC. These instructions can be found at http://www.orei.qut.edu.au/human/. < The CI/PS will submit any necessary reports related to the safety of

Appendices 120

research participants in accordance with UHREC policy and procedures. These instructions can be found at http://www.orei.qut.edu.au/human/. < The CI/PS will report to the UHREC annually in the specified format and notify the UHREC when the project is completed at all sites. < The CI/PS will notify the UHREC if the project is discontinued at a participating site before the expected completion date, with reasons provided. < The CI/PS will notify the UHREC of any plan to extend the duration of the project past the approval period listed above and will submit any associated required documentation. Instructions for obtaining an extension of approval can be found at http://www.orei.qut.edu.au/human/. < The CI/PS will notify the UHREC of his or her inability to continue as CI/PS including the name of and contact information for a replacement. This email constitutes ethics approval only. If appropriate, please ensure the appropriate authorisations are obtained from the institutions, organisations or agencies involved in the project and/or where the research will be conducted. The UHREC Terms of Reference, Standard Operating Procedures, membership and standard forms are available from: http://www.orei.qut.edu.au/human/manage/conditions.jsp. Should you have any queries about the UHREC's consideration of your project please contact the Research Ethics Advisory Team on 07 3138 5123 or email [email protected]. The UHREC wishes you every success in your research. Research Ethics Advisory Team, Office of Research Ethics & Integrity on behalf of the Chair, UHREC Level 4 | 88 Musk Avenue | Kelvin Grove +61 7 3138 5123 | [email protected] The UHREC is constituted and operates in accordance with the National Statement on Ethical Conduct in Human Research (2007) and registered by the National Health and Medical Research Council (# EC00171).

Appendices 121

Appendix B ESSA Pre-Exercise Screening Tool

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Appendix C Adverse Event Report

Dear Dr Andrew Hunt Ethics Number: 1800000438 Clearance Until: 1/08/2019 Ethics Category: Human This email is to advise that your adverse event submission has been reviewed by the Chair, University Human Research Ethics Committee. The Chair has advised the following: Location/date: QUT Kelvin Grove O‐A214 18/03/2019 Summary: Participant is 23 yo female, with no self‐identified health risks, recreationally active through distance trail running. She passed the aerobic fitness assessment and Exercise Sports Science Australia Pre‐Exercise Screening Tool. During the second of 4 walking events (walking on a treadmill wearing a helmet and weighted vest in a hot environment, each separated by 30 minutes of rest) the participant was noticeably fatigued. The participant’s physiological measurements leading up to the event were not indicative of any major strain on her body. Her heart rate and core temperature were well within normal limits, and she was talkative and comfortable right up until the events the researcher asked if they were OK to continue the participant stumbled, but was able to control the stumble through holding on to the hand rails. Action taken: The research team immediately followed the adverse event action plan: 1.Stopped the test. 2. Allowed the participant to recover in a supine position. 3. Elevated their legs and fan cooling. 4. Monitored physiological strain returning to baseline. 5. Verbally and physiologically confirmed that the participant was recovered before allowing them to leave. Current situation: The research team contacted the participant that evening after the adverse event. The participant reported feeling a bit iffy and thought they might be coming down with a cold or something. The researcher recommended visiting their GP if symptoms progressed. The research team used this opportunity to revisit the action plan and remain happy that it is appropriate and well understood by the research team. UHREC Chair’s response: Thank you for notifying us of this adverse event. The care for the participant in this instance was paramount, and I commend the researchers for implementing their adverse event action plan. The researchers have followed up with the participant who reported that they did have a cold, did not need any further medical treatment and has recovered. Risk rating: Moderate No further action is required. The event will be reported to UHREC at its next meeting. You only be contacted again if the UHREC raises any questions. Should you have any further queries please do not hesitate to contact the Research Ethics Advisory Team if you have any queries.

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Best regards Research Ethics Advisory Team, Office of Research Ethics & Integrity on behalf of Chair UHREC Level 4 | 88 Musk Avenue | Kelvin Grove +61 7 3138 5123 [email protected]

http://www.orei.qut.edu.au

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